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/app-server-protocol/src/protocol/v2.rs

@@ -2241,7 +2241,8 @@ pub struct ListMcpServerStatusResponse {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct McpResourceReadParams {
-    pub thread_id: String,
+    #[ts(optional = nullable)]
+    pub thread_id: Option<String>,
     pub server: String,
     pub uri: String,
 }