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/core/src/config/schema_tests.rs

@@ -53,3 +53,23 @@ Run `just write-config-schema` to overwrite with your changes.\n\n{diff}"
         "fixture should match exactly with generated schema"
     );
 }
+
+#[test]
+fn config_schema_hides_unsupported_inline_mcp_bearer_token() {
+    let schema_json = config_schema_json().expect("serialize config schema");
+    let schema_value: serde_json::Value =
+        serde_json::from_slice(&schema_json).expect("decode schema json");
+    let properties = schema_value
+        .pointer("/definitions/RawMcpServerConfig/properties")
+        .expect("RawMcpServerConfig properties should exist")
+        .as_object()
+        .expect("RawMcpServerConfig properties should be an object");
+
+    assert_eq!(
+        (
+            properties.contains_key("bearer_token"),
+            properties.contains_key("bearer_token_env_var"),
+        ),
+        (false, true),
+    );
+}