Make MCP resource read threadless (#18292)

## Summary

Making thread id optional so that we can better cache resources for MCPs
for connectors since their resource templates is universal and not
particular to projects.

- Make `mcpServer/resource/read` accept an optional `threadId`
- Read resources from the current MCP config when no thread is supplied
- Keep the existing thread-scoped path when `threadId` is present
- Update the generated schemas, README, and integration coverage

## Testing
- `just write-app-server-schema`
- `just fmt`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-mcp`
- `cargo test -p codex-app-server --test all mcp_resource`
- `just fix -p codex-mcp`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`

codex-rs/codex-mcp/src/lib.rs

@@ -32,6 +32,7 @@ pub use mcp::group_tools_by_server;
 pub use mcp::mcp_permission_prompt_is_auto_approved;
 pub use mcp::oauth_login_support;
 pub use mcp::qualified_mcp_tool_name_prefix;
+pub use mcp::read_mcp_resource;
 pub use mcp::resolve_oauth_scopes;
 pub use mcp::should_retry_without_scopes;
 pub use mcp::split_qualified_tool_name;