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/schema/json/codex_app_server_protocol.schemas.json

@@ -9685,7 +9685,10 @@
             "type": "string"
           },
           "threadId": {
-            "type": "string"
+            "type": [
+              "string",
+              "null"
+            ]
           },
           "uri": {
             "type": "string"
@@ -9693,7 +9696,6 @@
         },
         "required": [
           "server",
-          "threadId",
           "uri"
         ],
         "title": "McpResourceReadParams",