chore(compat): remove dead chat_response_to_messages_response

[ax128]

What

Removes the unused chat_response_to_messages_response function and its sole consumer _ANTHROPIC_STOP_REASON_MAP from aegisgate/adapters/openai_compat/mapper.py.

Why

• Zero references anywhere in the tree (grep over *.py; not in any __all__; no dynamic/getattr use). The live non-stream messages←responses path uses responses_response_to_messages_response, not this Chat-based variant.
• The dead function is also latently buggy: it emits a text-only content block and silently drops tool_calls, and hardcodes a stop-reason map that ignores tool_calls. Leaving it invites accidental future wiring that would re-introduce a tool-loop break.

Scope

Pure dead-code deletion — single file, 41 deletions, no behavior change, no interface/shape change.

Validation

• grep confirms no remaining references to either symbol.
• ruff check mapper.py clean; mypy mapper.py → Success.
• Full local suite: 943 passed, 10 skipped — byte-for-byte identical to main. The one failing test (test_doc_alignment::test_filter_class_count_matches_docs) is pre-existing on main (reproduced there), caused by untracked local test fixtures (DummyFilter etc.) polluting the BaseFilter subclass count when the full on-disk suite runs; it passes in isolation and is not in the CI-tracked path. Not introduced by this PR.
• Remote CI (3.10/3.12/3.13): pending — not yet observed.

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