feat: add reasoning content to AssistantChatMessage and StreamDelta

[mwien]

Context

Closes SAP/ai-sdk-python-backlog#41.

What this PR does and why it is needed

The reasoning_content field was missing in two response classes: AssistantChatMessage and StreamDelta.

Definition of Done

• Code is tested (Unit, Integration, E2E)
  • NOTE: orchestration_v2/models/response.py has no unit tests all (unless I missed something). Because of the minimal change I did not add any tests for StreamDelta.
• Error handling created / updated & covered by the tests above
• Documentation updated
  • Only Public APIs are allowed to be used in documentation/tutorials/sample code
• (Optional) Aligned changes with the JS/TS and Java SDK
• (Optional) Release notes updated -->

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[yamaceay]

lgtm

[yamaceay]

approving again after fixing the merge conflict