Add completion and cancellation docs for AI Transport

[zknill]

Add a new page covering completion signalling and
cancellation patterns for AI responses streamed via message-per-response. The page explains content-part completion using operation metadata empty appends, response-level completion via discrete messages,
detecting completion from history, and user-initiated cancellation with correlation IDs.

Also improve the message-per-response page with:

• Required channel capabilities table
• Guidance on awaiting appends between content parts
• Clarification that data should be raw strings, not JSON-encoded
• Note on append conflation and message.update delivery
• Link to the new completion and cancellation page

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch zak/ait-improvements

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