Python: fix(devui): fallback to empty string when reasoning chunk has no text

[Hardik K (hardikkaurani)]

Description

Fixes #9533: When running an agent with Gemini (or any client where a reasoning chunk has no text content), content.text can be None. In MessageMapper._map_reasoning_content (agent_framework_devui/_mapper.py), passing delta=content.text causes Pydantic to raise:

1 validation error for ResponseReasoningTextDeltaEvent
delta
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]

This PR:

• Adds an empty string fallback delta=content.text or "" in _map_reasoning_content to ensure valid string input for ResponseReasoningTextDeltaEvent.
• Adds unit tests in python/packages/devui/tests/devui/test_mapper.py covering reasoning content mapping with both None and valid text.

Fixes #9533

cc Vincent Biret (@baywet) westey (@westey-m)

[Hardik K (hardikkaurani)]

Hi Vincent Biret (@baywet) westey (@westey-m), submitted this PR to address #9533 where reasoning chunks with no text content cause Pydantic validation failure in DevUI _map_reasoning_content. Added unit tests covering both None and valid text scenarios. Ready for your review, thank you!

[Copilot]

🟡 Changes recommended

Both new tests initialize the wrong sequence key and raise KeyError before reaching their assertions.

1 open issue

• Initialize sequence_counter in test contexts · New

_{Review effort: Balanced}

[Hardik K (hardikkaurani)]

Vincent Biret (@baywet) The requested test-context fix is now in 04baa1e. Both reasoning-content tests initialize sequence_counter correctly, and local Python 3.11 DevUI tests plus targeted prek checks pass. Thank you for the review.

[Vincent Biret (baywet)]

Thank you for making the changes!

[Copilot]

🟢 Approval recommended

The focused fix addresses the reported failure, and the tests correctly initialize the required sequence counter.

No open issues 🎉

1 resolved since last review

• Initialize sequence_counter in test contexts

_{Review effort: Balanced}

[Hardik K (hardikkaurani)]

appreciated for the review Vincent Biret (@baywet)