feat: experimental flags (#10231)

## Problem being solved
- We need a single, reliable way to mark app-server API surface as
experimental so that:
  1. the runtime can reject experimental usage unless the client opts in
2. generated TS/JSON schemas can exclude experimental methods/fields for
stable clients.

Right now that’s easy to drift or miss when done ad-hoc.

## How to declare experimental methods and fields
- **Experimental method**: add `#[experimental("method/name")]` to the
`ClientRequest` variant in `client_request_definitions!`.
- **Experimental field**: on the params struct, derive `ExperimentalApi`
and annotate the field with `#[experimental("method/name.field")]` + set
`inspect_params: true` for the method variant so
`ClientRequest::experimental_reason()` inspects params for experimental
fields.

## How the macro solves it
- The new derive macro lives in
`codex-rs/codex-experimental-api-macros/src/lib.rs` and is used via
`#[derive(ExperimentalApi)]` plus `#[experimental("reason")]`
attributes.
- **Structs**:
- Generates `ExperimentalApi::experimental_reason(&self)` that checks
only annotated fields.
  - The “presence” check is type-aware:
    - `Option<T>`: `is_some_and(...)` recursively checks inner.
    - `Vec`/`HashMap`/`BTreeMap`: must be non-empty.
    - `bool`: must be `true`.
    - Other types: considered present (returns `true`).
- Registers each experimental field in an `inventory` with `(type_name,
serialized field name, reason)` and exposes `EXPERIMENTAL_FIELDS` for
that type. Field names are converted from `snake_case` to `camelCase`
for schema/TS filtering.
- **Enums**:
- Generates an exhaustive `match` returning `Some(reason)` for annotated
variants and `None` otherwise (no wildcard arm).
- **Wiring**:
- Runtime gating uses `ExperimentalApi::experimental_reason()` in
`codex-rs/app-server/src/message_processor.rs` to reject requests unless
`InitializeParams.capabilities.experimental_api == true`.
- Schema/TS export filters use the inventory list and
`EXPERIMENTAL_CLIENT_METHODS` from `client_request_definitions!` to
strip experimental methods/fields when `experimental_api` is false.

defs.bzl

@@ -1,7 +1,7 @@
 load("@crates//:data.bzl", "DEP_DATA")
 load("@crates//:defs.bzl", "all_crate_deps")
 load("@rules_platform//platform_data:defs.bzl", "platform_data")
-load("@rules_rust//rust:defs.bzl", "rust_binary", "rust_library", "rust_test")
+load("@rules_rust//rust:defs.bzl", "rust_binary", "rust_library", "rust_proc_macro", "rust_test")
 load("@rules_rust//cargo/private:cargo_build_script_wrapper.bzl", "cargo_build_script")
 
 PLATFORMS = [
@@ -34,6 +34,7 @@ def codex_rust_crate(
         crate_features = [],
         crate_srcs = None,
         crate_edition = None,
+        proc_macro = False,
         build_script_data = [],
         compile_data = [],
         lib_data_extra = [],
@@ -63,6 +64,7 @@ def codex_rust_crate(
         crate_srcs: Optional explicit srcs; defaults to `src/**/*.rs`.
         crate_edition: Rust edition override, if not default.
             You probably don't want this, it's only here for a single caller.
+        proc_macro: Whether this crate builds a proc-macro library.
         build_script_data: Data files exposed to the build script at runtime.
         compile_data: Non-Rust compile-time data for the library target.
         lib_data_extra: Extra runtime data for the library target.
@@ -109,7 +111,8 @@ def codex_rust_crate(
         deps = deps + [name + "-build-script"]
 
     if lib_srcs:
-        rust_library(
+        lib_rule = rust_proc_macro if proc_macro else rust_library
+        lib_rule(
             name = name,
             crate_name = crate_name,
             crate_features = crate_features,