feat(api): add POST /state endpoint to update session state

[triangle-z]

This PR introduces a new endpoint as described in #1117 :

POST /apps/{app_name}/users/{user_id}/sessions/{session_id}/state

It allows external tools, UIs, or services to update session state via the API programmatically, without needing to send a user message.

I added:

• New FastAPI route update_session_state
• Support for append_event in MockSessionService
• Tests

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Example usage:

curl -X POST http://localhost:8080/apps/my_app/users/u1/sessions/s1/state \
     -H 'Content-Type: application/json' \
     -d '{"foo": "bar"}'

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Pytest Comparison:

Main Branch (Before PR)

Feature Branch (This PR)

All new tests passed, and no test failed that didn't fail in the main branch.