fix(deps): bump h2 to 4.4.1 for CVE-2026-71554 (#2839)

## Description

`pip-audit` is currently red on every open PR. Not because of anything
in those branches — `uv.lock` pins `h2` at 4.3.0, and CVE-2026-71554 was
published against `h2 <=4.4.0`.

> h2 <=4.4.0 accepts request header blocks containing more than one Host
header, and forwards every Host header to the consuming application.
Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request
carries two Host header lines, which is a request smuggling primitive
(CWE-444).

Fixed in 4.4.1.

Closes #

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- `uv lock --upgrade-package h2`, which moves exactly two packages:

  ```
  h2     4.3.0 -> 4.4.1
  hpack  4.1.0 -> 4.2.0
  ```

`h2` arrives transitively via `httpx[http2]`, and the constraint in
`pyproject.toml` is already wide enough (`>=3,<5`), so only the lock
needed to move — no source or `pyproject.toml` change.
`requirements-prod.txt` is not checked in; the audit workflow exports it
from `uv.lock` at run time, so the lock bump is the entire fix.

## Testing

- [x] Unit tests pass (`pytest`)
- [ ] Linting passes (`ruff check .`)
- [ ] Type checking passes (`mypy headroom`)
- [ ] New tests added for new functionality
- [x] Manual testing performed

### Test Output

Reproduced the CI gate locally with the exact command from
`.github/workflows/security.yml`:

```text
$ uv export --frozen --no-dev --no-emit-project --no-hashes \
    --extra all --format requirements-txt > requirements-prod.txt

$ grep -E '^(h2|hpack)==' requirements-prod.txt
h2==4.4.1
hpack==4.2.0

$ pip-audit -r requirements-prod.txt
No known vulnerabilities found
```

Before this change, the same command reported:

```text
Name | Version | ID              | Fix Versions
h2   | 4.3.0   | CVE-2026-71554  | 4.4.1
Found 1 known vulnerability in 1 package
```

## Real Behavior Proof

- **Environment:** macOS, uv 0.9.x, Python 3.12.6.
- **Exact command / steps:** `uv lock --upgrade-package h2 --dry-run` to
confirm the blast radius, then the real lock, then the workflow's own
export + `pip-audit` invocation.
- **Observed result:** resolution touches only `h2` and `hpack`; 269
packages resolved with no other version movement. `pip-audit` goes from
1 known vulnerability to none.
- **Not tested:** HTTP/2 traffic against a live upstream. `h2` 4.4.1 is
a patch release on a library used transitively by `httpx`; Headroom does
not import `h2` directly (`grep -rn "import h2" headroom/` is empty), so
the exposure is whatever `httpx[http2]` does with it.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] I did **not** edit `CHANGELOG.md`

## Additional Notes

N/A items above: no code changed, so ruff/mypy/new tests do not apply —
the verification that matters is the audit output, which is quoted in
full.

**Why this is standalone.** It surfaced while fixing CI on #2838, but it
is not caused by that branch and it blocks #2832 identically. Landing it
separately unblocks the gate for every open PR at once and keeps a
supply-chain bump out of an unrelated change.

**One unrelated warning the resolver prints**, noted so it is not
mistaken for a side effect of this PR:

```
warning: `pypdfium2==5.12.0` is yanked (reason: "Setup blunder breaking some
bindgen codepaths ... Wheels are valid and effectively identical to 5.12.1")
```

That predates this change and is not touched by it. Worth its own bump,
but not here.

uv.lock

@@ -1667,15 +1667,15 @@ wheels = [
 
 [[package]]
 name = "h2"
-version = "4.3.0"
+version = "4.4.1"
 source = { registry = "https://pypi.org/simple/" }
 dependencies = [
     { name = "hpack" },
     { name = "hyperframe" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/1d/17/afa56379f94ad0fe8defd37d6eb3f89a25404ffc71d4d848893d270325fc/h2-4.3.0.tar.gz", hash = "sha256:6c59efe4323fa18b47a632221a1888bd7fde6249819beda254aeca909f221bf1", size = 2152026, upload-time = "2025-08-23T18:12:19.778Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/e7/85/7c366e69d84c17bb778fe41419e1fbcce3033d5b7ce29bbffff0a98b859f/h2-4.4.1.tar.gz", hash = "sha256:4e866ffb1a869ae14dd9b5e6beb5c24a13da0495ad72b65925ded182521c1516", size = 2157281, upload-time = "2026-08-03T11:45:09.509Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/69/b2/119f6e6dcbd96f9069ce9a2665e0146588dc9f88f29549711853645e736a/h2-4.3.0-py3-none-any.whl", hash = "sha256:c438f029a25f7945c69e0ccf0fb951dc3f73a5f6412981daee861431b70e2bdd", size = 61779, upload-time = "2025-08-23T18:12:17.779Z" },
+    { url = "https://files.pythonhosted.org/packages/7e/22/e85faf23bd72a92d1921e37d674ca56eb298a3c8be31fdecef0ff2b3aaac/h2-4.4.1-py3-none-any.whl", hash = "sha256:0e25f1462b23c9cb82d9eb02e28bc706dac2a68cb457c6a0d74d63c8a2a5d0e6", size = 62636, upload-time = "2026-08-03T11:44:59.164Z" },
 ]
 
 [[package]]
@@ -2078,11 +2078,11 @@ sdist = { url = "https://files.pythonhosted.org/packages/cf/7a/1a9b1405f2eb59515
 
 [[package]]
 name = "hpack"
-version = "4.1.0"
+version = "4.2.0"
 source = { registry = "https://pypi.org/simple/" }
-sdist = { url = "https://files.pythonhosted.org/packages/2c/48/71de9ed269fdae9c8057e5a4c0aa7402e8bb16f2c6e90b3aa53327b113f8/hpack-4.1.0.tar.gz", hash = "sha256:ec5eca154f7056aa06f196a557655c5b009b382873ac8d1e66e79e87535f1dca", size = 51276, upload-time = "2025-01-22T21:44:58.347Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/26/5b/fcabf6028144a8723726318b07a32c2f3314acdff6265743cf08a344b18e/hpack-4.2.0.tar.gz", hash = "sha256:0895cfa3b5531fc65fe439c05eb65144f123bf7a394fcaa56aa423548d8e45c0", size = 51300, upload-time = "2026-06-23T18:34:46.667Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/07/c6/80c95b1b2b94682a72cbdbfb85b81ae2daffa4291fbfa1b1464502ede10d/hpack-4.1.0-py3-none-any.whl", hash = "sha256:157ac792668d995c657d93111f46b4535ed114f0c9c8d672271bbec7eae1b496", size = 34357, upload-time = "2025-01-22T21:44:56.92Z" },
+    { url = "https://files.pythonhosted.org/packages/71/b4/4a9fcfb2aef6ba44d9073ecd301443aa00b3dac95de5619f2a7de7ec8a91/hpack-4.2.0-py3-none-any.whl", hash = "sha256:858ac0b02280fa582b5080d68db0899c62a80375e0e5413a74970c5e518b6986", size = 34246, upload-time = "2026-06-23T18:34:45.472Z" },
 ]
 
 [[package]]