code_mode: Move exec params from runtime declarations to @pragma (#14511)

This change moves code_mode exec session settings out of the runtime API
and into an optional first-line pragma, so instead of calling runtime
helpers like set_yield_time() or set_max_output_tokens_per_exec_call(),
the model can write // @exec: {"yield_time_ms": ...,
"max_output_tokens": ...} at the top of the freeform exec source. Rust
now parses that pragma before building the source, validates it, and
passes the values directly in the exec start message to the code-mode
broker, which applies them at session start without any worker-runtime
mutation path. The @openai/code_mode module no longer exposes those
setter functions, the docs and grammar were updated to describe the
pragma form, and the existing code_mode tests were converted to use
pragma-based configuration instead.

codex-rs/core/src/tools/spec.rs

@@ -2018,8 +2018,13 @@ fn create_js_repl_reset_tool() -> ToolSpec {
 
 fn create_code_mode_tool(enabled_tool_names: &[String]) -> ToolSpec {
     const CODE_MODE_FREEFORM_GRAMMAR: &str = r#"
-start: source
-source: /[\s\S]+/
+start: pragma_source | plain_source
+pragma_source: PRAGMA_LINE NEWLINE SOURCE
+plain_source: SOURCE
+
+PRAGMA_LINE: /[ \t]*\/\/ @exec:[^\r\n]*/
+NEWLINE: /\r?\n/
+SOURCE: /[\s\S]+/
 "#;
 
     ToolSpec::Freeform(FreeformTool {