[Platform] Serialize assistant message content as content parts in Open Responses bridge

[chr-hertel]

Q	A
Bug fix?	yes
New feature?	no
Docs?	no
Issues	Fix #2346
License	MIT

• AssistantMessageNormalizer emitted content as a raw string for text-only assistant messages; strict Responses API implementations (e.g. litellm) reject that and expect an output message item
• Now emits the fully-specified output message item: type: message, id (derived from the message UUID), status: completed, and content as output_text parts with annotations — the shape strict validators (litellm/Pydantic) require, since a partial item matches neither EasyInputMessageParam nor ResponseOutputMessageParam
• Verified live against OpenAI, Azure OpenAI, and Scaleway (which runs a strict litellm-style validator and rejected both the old raw string and intermediate partial shapes)
• Added contract-level regression test reproducing the message bag from the issue

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