Exposes final_output_json_schema through the app-server protocol

[tomasgvivo]

Changes

• Added the optional final_output_json_schema field to the app-server protocol’s user-turn payload so callers can supply
  a JSON schema with each turn (app-server-protocol/src/protocol.rs:543).
• Plumbed the new field through the app server so the conversation submission passes the schema down to core when
  present (app-server/src/codex_message_processor.rs:1008).
• Expanded the send-user-turn flow tests to populate the new field explicitly, keeping existing behavioural
  assertions intact (app-server/tests/suite/codex_message_processor_flow.rs:337, app-server/tests/suite/
  codex_message_processor_flow.rs:470, app-server/tests/suite/codex_message_processor_flow.rs:506).
• Added focused protocol serialization tests that confirm the schema is emitted only when provided (app-server-protocol/
  src/protocol.rs:935, app-server-protocol/src/protocol.rs:965).

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[tomasgvivo]

I have read the CLA Document and I hereby sign the CLA

[tomasgvivo]

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