app-server: treat null mode developer instructions as built-in defaults (#10983)

## Summary
- make `turn/start` normalize
`collaborationMode.settings.developer_instructions: null` to the
built-in instructions for the selected mode
- prevent app-server clients from accidentally clearing mode-switch
developer instructions by sending `null`
- document this behavior in the v2 protocol and app-server docs

## What changed
- `codex-rs/app-server/src/codex_message_processor.rs`
  - added a small `normalize_turn_start_collaboration_mode` helper
  - in `turn_start`, apply normalization before `OverrideTurnContext`
- `codex-rs/app-server/tests/suite/v2/turn_start.rs`
- extended `turn_start_accepts_collaboration_mode_override_v2` to assert
the outgoing request includes default-mode instruction text when the
client sends `developer_instructions: null`
- `codex-rs/app-server-protocol/src/protocol/v2.rs`
- clarified `TurnStartParams.collaboration_mode` docs:
`settings.developer_instructions: null` means use built-in mode
instructions
- regenerated schema fixture:
- `codex-rs/app-server-protocol/schema/typescript/v2/TurnStartParams.ts`
- docs:
  - `codex-rs/app-server/README.md`
  - `codex-rs/docs/codex_mcp_interface.md`

codex-rs/docs/codex_mcp_interface.md

@@ -112,6 +112,8 @@ Fetch the built-in collaboration mode presets with `collaborationMode/list`. Thi
 - `data` – ordered list of collaboration mode masks (partial settings to apply on top of the base mode)
   - For tri-state fields like `reasoning_effort` and `developer_instructions`, omit the field to keep the current value, set it to `null` to clear it, or set a concrete value to update it.
 
+When sending `turn/start` with `collaborationMode`, `settings.developer_instructions: null` means "use built-in instructions for the selected mode".
+
 ## Event stream
 
 While a conversation runs, the server sends notifications: