Let extensions process MCP tool results (#41202)

## What changed

- Add `ToolLifecycleContributor::on_mcp_tool_result` with access to the
  executed MCP tool context, rewritten arguments, extension data stores, and
  mutable server result.
- Run contributors before publishing MCP completion and before preparing the
  result for the model, so extensions can inspect or replace successful and
  error results.

## Testing

- Cover unchanged, replaced, and error results for direct MCP calls, plus
  replaced and error results through Code Mode.
- Verify completion waits for result processing and the processed result flows
  to both completion events and subsequent model input.

GitOrigin-RevId: ece7acfe9c5eed4778cb78a29c3ffd06a74e1bcf

codex-rs/ext/extension-api/src/lib.rs

@@ -51,6 +51,7 @@ pub use contributors::McpServerContribution;
 pub use contributors::McpServerContributionContext;
 pub use contributors::McpServerContributor;
 pub use contributors::McpToolContext;
+pub use contributors::McpToolResultInput;
 pub use contributors::McpToolSource;
 pub use contributors::PreviousWorldStateSection;
 pub use contributors::PromptFragment;