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/app-server-protocol/src/protocol/v2.rs

@@ -2089,6 +2089,9 @@ pub struct TurnStartParams {
 
     /// EXPERIMENTAL - Set a pre-set collaboration mode.
     /// Takes precedence over model, reasoning_effort, and developer instructions if set.
+    ///
+    /// For `collaboration_mode.settings.developer_instructions`, `null` means
+    /// "use the built-in instructions for the selected mode".
     #[experimental("turn/start.collaborationMode")]
     #[ts(optional = nullable)]
     pub collaboration_mode: Option<CollaborationMode>,