Hide unsupported MCP bearer_token from config schema (#19294)

## Summary

Fixes #19275.

Codex runtime rejects inline MCP `bearer_token` config entries and asks
users to configure `bearer_token_env_var` instead, but the generated
config schema still advertised `mcp_servers.<name>.bearer_token` as a
supported field. That made editor/schema validation disagree with
runtime validation.

This keeps `bearer_token` in `RawMcpServerConfig` so Codex can continue
producing the targeted runtime error for recent or existing configs, but
skips the field during schemars generation. The checked-in
`core/config.schema.json` fixture now exposes `bearer_token_env_var`
without exposing unsupported inline `bearer_token`.

## Verification

- Added `config_schema_hides_unsupported_inline_mcp_bearer_token` to
assert the generated schema hides `bearer_token` while preserving
`bearer_token_env_var`.
- Ran `cargo test -p codex-config`.
- Ran `cargo test -p codex-core config_schema`.

codex-rs/config/src/mcp_types.rs

@@ -176,7 +176,11 @@ pub struct McpServerConfig {
     pub tools: HashMap<String, McpServerToolConfig>,
 }
 
-/// Raw MCP config shape used for deserialization and JSON Schema generation.
+/// Raw MCP config shape used for deserialization and supported-field JSON
+/// Schema generation.
+///
+/// Fields that are accepted only to produce targeted validation errors should
+/// be skipped in the generated schema.
 ///
 /// Keep `TryFrom<RawMcpServerConfig> for McpServerConfig` exhaustively
 /// destructuring this struct so new TOML fields cannot be added here without
@@ -200,6 +204,7 @@ pub struct RawMcpServerConfig {
 
     // streamable_http
     pub url: Option<String>,
+    #[schemars(skip)]
     pub bearer_token: Option<String>,
     pub bearer_token_env_var: Option<String>,