fix(transforms/smart_crusher): don't crash on a tool call with a null function (#2232)

## Description

A tool call whose `function` field is explicitly `null` crashes
SmartCrusher's per-request context extraction.

`_extract_context_from_messages` (called at the top of `apply()`) walks
recent assistant tool calls:

```python
for tc in msg.get("tool_calls", []):
    if isinstance(tc, dict):
        func = tc.get("function", {})
        args = func.get("arguments", "")
```

`dict.get("function", {})` only substitutes `{}` when the key is
**missing**. When the key is present but `null` — `{"id": "1", "type":
"function", "function": null}`, which clients emit for a partial or
streamed tool call — `func` is `None`, and `None.get("arguments")`
raises `AttributeError`. That propagates out of
`_extract_context_from_messages` and crashes `apply()` for the entire
request, so the request either errors or has to fail open to
uncompressed with a logged traceback.

The sibling `_build_tool_name_index` in the same file already guards
this exact shape with `(tc.get("function") or {})` — this call site just
wasn't updated to match.

## Fix

Use the same null-safe form:

```python
func = tc.get("function") or {}
```

`None` (and any other falsy value) now collapses to `{}`, the null tool
call contributes no context, and extraction continues to the next call.

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

- `headroom/transforms/smart_crusher.py`: `tc.get("function", {})` →
`tc.get("function") or {}` in `_extract_context_from_messages`.
- `tests/test_transforms/test_smart_crusher_bugs.py`: new test asserting
a `{"function": null}` tool call doesn't crash extraction and later
calls are still read.
- `CHANGELOG.md`: Bug Fixes entry.

## Testing

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

### Test Output

```text
$ uvx ruff@0.15.17 check headroom/transforms/smart_crusher.py tests/test_transforms/test_smart_crusher_bugs.py
All checks passed!
$ uvx mypy@1.20.2 --ignore-missing-imports headroom/transforms/smart_crusher.py
Success: no issues found in 1 source file
```

## Real Behavior Proof

- Environment: Windows 11, Python 3.12, `uvx ruff@0.15.17` / `uvx
mypy@1.20.2`. A full `pytest` OOM-kills this box (ML stack import), so I
reproduced the extraction loop with a dependency-free script and left
the full pytest to CI.
- Exact command / steps: ran an assistant message with tool calls
`[{"function": null}, {"function": {"arguments": "keep-me"}}]` through
the OLD `get("function", {})` loop and the NEW `get("function") or {}`
loop.
- Observed result: OLD raises `AttributeError` on the null function; NEW
skips it and returns `"keep-me"` from the following call.
- Not tested: a live proxy request carrying a null-function tool call;
full local `pytest` deferred to CI (OOM).

## 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
- [x] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [x] I have updated the CHANGELOG.md if applicable

## Additional Notes

The "unit tests pass locally" box is unchecked because the full suite
imports the ML stack, which I can't run here. The new test uses the
existing `_make_crusher` helper in
`tests/test_transforms/test_smart_crusher_bugs.py`, so it runs under the
normal CI pytest job; behaviour is additionally verified by the
standalone proof above.

---------

Co-authored-by: JerrettDavis <mxjerrett@gmail.com>

headroom/transforms/smart_crusher.py

@@ -1202,7 +1202,12 @@ class SmartCrusher(Transform):
             if msg.get("role") == "assistant" and msg.get("tool_calls"):
                 for tc in msg.get("tool_calls", []):
                     if isinstance(tc, dict):
-                        func = tc.get("function", {})
+                        # `tc.get("function", {})` returns None for an explicit
+                        # {"function": null} (the default only applies to a
+                        # missing key), and `.get` on None raises AttributeError,
+                        # crashing apply(). Use the null-safe form the sibling
+                        # `_build_tool_name_index` already uses (line ~118).
+                        func = tc.get("function") or {}
                         args = func.get("arguments", "")
                         if isinstance(args, str) and args:
                             context_parts.append(args)

tests/test_transforms/test_smart_crusher_bugs.py

@@ -163,6 +163,26 @@ class TestLosslessOnlyMode:
         assert json.loads(out.compressed) == rows
 
 
+def test_extract_context_survives_null_function_tool_call() -> None:
+    # A tool_call with an explicit {"function": null} must not crash context
+    # extraction: `dict.get("function", {})` returns None for a present-but-null
+    # key, and `.get` on None raises AttributeError inside apply().
+    crusher = _make_crusher()
+    messages = [
+        {
+            "role": "assistant",
+            "tool_calls": [
+                {"id": "1", "type": "function", "function": None},
+                {"id": "2", "type": "function", "function": {"arguments": "keep-me"}},
+            ],
+        },
+    ]
+
+    ctx = crusher._extract_context_from_messages(messages)
+
+    assert "keep-me" in ctx
+
+
 # Stage 3c.1 lockstep bug-fix tests previously lived here; they probed
 # Python helpers (`_percentile_linear`, `_detect_sequential_pattern`,
 # `_detect_rare_status_values`, `_compute_k_split`) that were removed