## Description
Buffered Anthropic `/v1/messages` requests still use Headroom's generic
300-second read timeout, which can produce proxy-generated `502
ReadTimeout` errors on long turns. This adds a dedicated buffered
Anthropic timeout, keeps it applied across CCR and memory continuations
plus batch paths, and makes the direct server entrypoint enforce the
same positive-integer contract as the Click CLI. Closes#1261.
## Type of Change
- [x] Bug fix (non-breaking change that fixes an issue)
## Changes Made
- Added `anthropic_buffered_request_timeout_seconds` for buffered
Anthropic reads.
- Routed `/v1/messages`, CCR continuation, memory continuation, batch
create, batch passthrough, and batch results through that timeout.
- Enforced the same positive-integer validation for
`HEADROOM_ANTHROPIC_BUFFERED_REQUEST_TIMEOUT_SECONDS` and
`--anthropic-buffered-request-timeout-seconds` in both startup paths.
- Added focused regressions and updated `CHANGELOG.md`.
## Testing
- [x] `uv run pytest tests/test_proxy/test_anthropic_buffered_timeout.py
tests/test_cli_proxy_improvements.py::TestNewEnvVarWiring`
- [x] `uv run ruff check .`
- [x] `uv run ruff format . --check`
### Test Output
```text
$ uv run pytest tests/test_proxy/test_anthropic_buffered_timeout.py tests/test_cli_proxy_improvements.py::TestNewEnvVarWiring
17 passed in 3.42s
$ uv run ruff check .
All checks passed!
$ uv run ruff format . --check
966 files already formatted
```
## Real Behavior Proof
- Environment: local FastAPI `TestClient` with stubbed retry and HTTP
client seams
- Exact command / steps: run `uv run pytest
tests/test_proxy/test_anthropic_buffered_timeout.py
tests/test_cli_proxy_improvements.py::TestNewEnvVarWiring`; the tests
build `ProxyConfig(request_timeout_seconds=7, connect_timeout_seconds=3,
anthropic_buffered_request_timeout_seconds=19)`, drive `/v1/messages`,
`/v1/messages/batches`, `/v1/messages/batches/{batch_id}/results`, a CCR
continuation, and a memory continuation through `TestClient`, then
verify `HEADROOM_ANTHROPIC_BUFFERED_REQUEST_TIMEOUT_SECONDS=0` falls
back to `600`, `--anthropic-buffered-request-timeout-seconds 0` is
rejected, and default proxy timeouts stay `read=300` and `write=300`
- Observed result: buffered Anthropic paths use
`httpx.Timeout(connect=3, read=19, write=7, pool=3)`, continuation
requests stay on that same budget, invalid zero-valued startup config is
rejected or ignored back to the default, and unrelated proxy timeout
defaults stay unchanged
- Not tested: live upstream Anthropic latency beyond the focused
stubbed-timeout regression
## 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 added tests that prove the fix
- [x] New and existing unit tests pass locally with my changes
- [x] I have updated the CHANGELOG.md if applicable
