test(agno): follow the metrics dataclass move in agno 3.0.0 (#3260)

## Description

agno 3.0.0 (released 2026-08-24) removed the `agno.models.metrics`
module; the per-message usage dataclass now lives at `agno.metrics`
under the name `MessageMetrics`. The mock fixtures in
`tests/test_integrations/agno/test_model.py` import the old path inline,
and the `test-agno` CI job installs `wheel[dev,agno]` with an unpinned
`agno>=1.0.0`, so it now resolves agno 3.0.0 and fails on every branch -
including `main` (see the CI run for #3239's merge commit) and
currently-open PRs.

This resolves the class once at module level: prefer the pre-3 location,
fall back to `MessageMetrics` on agno >= 3. `MessageMetrics` exists
under both names in 2.x and the constructor kwargs the fixtures use
(`input_tokens`, `output_tokens`, `total_tokens`) are unchanged, so both
major versions stay green. Tests-only change; the runtime integration
(`headroom/integrations/agno/`) never imported the removed module - the
other 76 agno tests already pass on 3.0.0.

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)

## Changes Made

- Replace the two inline `from agno.models.metrics import Metrics`
imports in the `mock_agno_model` fixture with one module-level compat
resolution that tries `agno.models.metrics.Metrics` (agno < 3) and falls
back to `agno.metrics.MessageMetrics as Metrics` (agno >= 3).

## Testing

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

### Test Output

```text
$ uv run --frozen --extra dev --extra agno --with agno==3.0.0 pytest tests/test_integrations/agno/ -q
================== 79 passed, 5 skipped, 1 warning in 19.87s ===================

$ uv run --frozen --extra dev --extra agno --with agno==2.9.0 pytest tests/test_integrations/agno/ -q
================== 79 passed, 5 skipped, 1 warning in 11.60s ===================

$ ruff check tests/test_integrations/agno/test_model.py
All checks passed!
$ ruff format --check tests/test_integrations/agno/test_model.py
1 file already formatted
```

## Real Behavior Proof

- Environment: macOS 15 (arm64), CPython 3.12, uv-managed venv; branch =
upstream/main `6262c28a` + this one test commit.
- Exact command / steps: On pristine upstream/main, `uv run --frozen
--extra dev --extra agno --with agno==3.0.0 pytest
tests/test_integrations/agno/ -q` reproduces the CI failure: 3 failed
(`test_response_applies_optimization`,
`test_response_stream_applies_optimization`,
`test_model_wrapper_real_optimization`), all `ModuleNotFoundError: No
module named 'agno.models.metrics'` - the same three failures as the
`test-agno` job on current PRs. Applied this commit and re-ran the same
command under agno 3.0.0 and agno 2.9.0.
- Observed result: 79 passed / 5 skipped under both agno versions; the
three fail-before tests pass.
- Not tested: agno 1.x (the extra's floor); `mypy headroom` not re-run -
no runtime module is touched.

## Runtime Rollout Safety

- Rollout-managed feature(s): None - tests only.
- Minimum rollout channel: Stable.
- Stable/default behavior changed: No. No shipped code changes.
- Kill switch / disable path: Not applicable (test-only change).
- Unsafe override required: No.
- Qualification impact: None.
- Rollback path: Revert the single commit.

## 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
- [x] 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` - it is generated by
release-please from my Conventional Commit PR title (a CI guard enforces
this)

## Additional Notes

- No new regression test: the three existing tests are the regression -
they fail on agno 3.0.0 without this change and pass with it. Docs
untouched (test-only fix).
- An alternative was pinning `agno<3` in the extra; not taken, since the
runtime integration works unmodified on 3.0.0 and a pin would block
users already on agno 3.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/test_integrations/agno/test_model.py

@@ -19,6 +19,11 @@ try:
     AGNO_AVAILABLE = True
 except ImportError:
     AGNO_AVAILABLE = False
+else:
+    try:  # agno < 3: the per-message usage dataclass lived at agno.models.metrics
+        from agno.models.metrics import Metrics
+    except ImportError:  # agno >= 3 moved it to agno.metrics, renamed MessageMetrics
+        from agno.metrics import MessageMetrics as Metrics
 
 from headroom import HeadroomConfig, HeadroomMode
 
@@ -50,8 +55,6 @@ def mock_agno_model():
 
     # Mock invoke method (returns ModelResponse for Agno's response() loop)
     def mock_invoke(messages, **kwargs):
-        from agno.models.metrics import Metrics
-
         # Create a proper ModelResponse that Agno's response() can process
         return ModelResponse(
             role="assistant",
@@ -73,8 +76,6 @@ def mock_agno_model():
 
     # Mock invoke_stream for streaming
     def mock_invoke_stream(messages, **kwargs):
-        from agno.models.metrics import Metrics
-
         yield ModelResponse(
             role="assistant",
             content="Streaming...",