Expose invocation sources to extension tools (#40382)

## What changed

- Add `ToolCallSource` to extension `ToolCall`s and propagate direct or Code Mode
  invocation metadata, including the runtime cell and nested tool-call IDs.
- Add `ToolCall::response_byte_budget` so extension tools can bound direct-call
  responses by the host truncation policy while allowing Code Mode responses up
  to the tool's own limit.
- Share the skills extension's response-size limit between its `list` and `read`
  tools.

## Testing

- Verify that Code Mode source metadata is preserved when core constructs an
  extension tool call.

GitOrigin-RevId: b10aef01be30d35b67bee3af24a1a4175c9ee112

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

@@ -27,6 +27,7 @@ pub use codex_tools::JsonToolOutput;
 pub use codex_tools::NoopTurnItemEmitter;
 pub use codex_tools::ResponsesApiTool;
 pub use codex_tools::ToolCall;
+pub use codex_tools::ToolCallSource;
 pub use codex_tools::ToolEnvironment;
 pub use codex_tools::ToolExecutor;
 pub use codex_tools::ToolExecutorFuture;
@@ -64,7 +65,6 @@ pub use contributors::ThreadStartInput;
 pub use contributors::ThreadStopInput;
 pub use contributors::TokenUsageContributor;
 pub use contributors::ToolCallOutcome;
-pub use contributors::ToolCallSource;
 pub use contributors::ToolContributor;
 pub use contributors::ToolFinishInput;
 pub use contributors::ToolLifecycleContributor;