Generate stable message IDs for Chat Completions streams - #128

Summary

• replace the Agents SDK __fake_id__ placeholder with a generated ChatKit message ID
• carry that ID through content-part, delta, annotation, and completion events
• add a regression test covering two consecutive Chat Completions messages

Testing

• focused synthetic-ID regression test
• pytest -q (131 tests)
• Ruff checks and formatting for changed files
• Pyright (0 errors)

Fixes #36