Python: fix: skip orphan Anthropic thinking signatures

[he-yufeng]

Fixes #5783. The Anthropic message serializer no longer emits thinking blocks with thinking set to null when a text_reasoning item only carries protected_data. If the signature immediately follows a thinking block, it is attached to that block; otherwise the orphan signature-only item is skipped before replay. Validation: uv run pytest packages/anthropic/tests/test_anthropic_client.py -k 'text_reasoning or signature_delta' -q; uv run ruff check packages/anthropic/agent_framework_anthropic/_chat_client.py packages/anthropic/tests/test_anthropic_client.py; uv run ruff format --check packages/anthropic/agent_framework_anthropic/_chat_client.py packages/anthropic/tests/test_anthropic_client.py; uv run pyright packages/anthropic/agent_framework_anthropic/_chat_client.py packages/anthropic/tests/test_anthropic_client.py; uv run mypy --config-file pyproject.toml packages/anthropic/agent_framework_anthropic; python -m py_compile; git diff --check. I also tried mypy on the full test file, but it reports existing unrelated type errors in packages/anthropic/tests/test_anthropic_client.py.

[Copilot]

Pull request overview

This PR fixes an Anthropic provider serialization bug where a signature-only text_reasoning content item (i.e., text=None with protected_data) could be emitted as an invalid Anthropic thinking block with thinking: null. The change ensures signature deltas are either attached to the preceding thinking block (when applicable) or skipped so replayed message history remains valid (per #5783).

Changes:

• Update _prepare_message_for_anthropic() to skip text_reasoning items with text is None, optionally attaching protected_data as a signature on the immediately preceding thinking block when it has no signature.
• Add unit tests covering (1) signature attachment to a prior thinking block and (2) skipping an orphan signature-only reasoning item before a tool call.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/packages/anthropic/agent_framework_anthropic/_chat_client.py	Prevents emitting invalid thinking: null blocks by attaching or skipping signature-only reasoning items during message preparation.
python/packages/anthropic/tests/test_anthropic_client.py	Adds regression tests for signature-only reasoning attachment and orphan skipping behavior.

[moonbox3]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/anthropic/agent_framework_anthropic
_chat_client.py	446	35	92%	461, 464, 545, 638, 640, 783, 819–820, 898, 900, 930–931, 976, 992–993, 1000–1002, 1006–1008, 1012–1015, 1129, 1139, 1191, 1339–1340, 1357, 1370, 1383, 1408–1409
TOTAL	33944	3909	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
6704	30 💤	0 ❌	0 🔥	1m 50s ⏱️

[he-yufeng]

Rebased this branch onto current upstream/main.

Validation:

$env:PYTHONPATH = "$(Resolve-Path python\packages\core);$(Resolve-Path python\packages\anthropic)"
python -m pytest python\packages\anthropic\tests\test_anthropic_client.py -q --basetemp .tmp\pytest-5784

python -m py_compile python\packages\anthropic\agent_framework_anthropic\_chat_client.py python\packages\anthropic\tests\test_anthropic_client.py
git diff --check

The first local run without PYTHONPATH failed during collection because the monorepo core package was not on sys.path; the command above is the passing run.