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/config.schema.json

@@ -1709,7 +1709,7 @@
     },
     "RawMcpServerConfig": {
       "additionalProperties": false,
-      "description": "Raw MCP config shape used for deserialization and JSON Schema generation.\n\nKeep `TryFrom<RawMcpServerConfig> for McpServerConfig` exhaustively destructuring this struct so new TOML fields cannot be added here without updating the validation/mapping logic that produces [`McpServerConfig`].",
+      "description": "Raw MCP config shape used for deserialization and supported-field JSON Schema generation.\n\nFields that are accepted only to produce targeted validation errors should be skipped in the generated schema.\n\nKeep `TryFrom<RawMcpServerConfig> for McpServerConfig` exhaustively destructuring this struct so new TOML fields cannot be added here without updating the validation/mapping logic that produces [`McpServerConfig`].",
       "properties": {
         "args": {
           "default": null,
@@ -1718,9 +1718,6 @@
           },
           "type": "array"
         },
-        "bearer_token": {
-          "type": "string"
-        },
         "bearer_token_env_var": {
           "type": "string"
         },