Make selected plugin roots URI-native (#28918)

## Why

Selected capability roots belong to the executor filesystem, not the
app-server host. Converting their path strings into the host's native
`Path` breaks whenever the two machines use different path conventions,
such as a Windows executor behind a Unix app-server.

This PR establishes `PathUri` as the selected-plugin boundary so the
executor remains authoritative for its paths.

## What changed

- Require `selectedCapabilityRoots[].location.path` to be a canonical
`file:` URI and deserialize it directly as `PathUri`; native path
strings are rejected.
- Update the app-server schema, generated TypeScript, examples, and
request coverage for the URI contract.
- Keep selected roots, resolved plugin locations, manifest paths, and
manifest resources as `PathUri`.
- Inspect and read plugin roots and manifests only through the selected
environment's `ExecutorFileSystem`.
- Parse executor manifests with the shared URI-native parser from #29620
instead of projecting them onto the host filesystem.
- Enforce resource containment lexically and preserve the root URI's
POSIX or Windows path convention.
- Cover foreign Windows plugin roots and URI-native manifest resources.

```text
thread/start
  selectedCapabilityRoots[].location.path = "file:///C:/plugins/demo"
                              | PathUri
                              v
                    ExecutorFileSystem
                              |
                              +--> plugin.json
                              +--> manifest resources
```

This PR stops at the shared selected-plugin representation. The next two
PRs remove the remaining host-path projections in the skill and MCP
consumers.

## Stack

1. #29614 — add lexical `PathUri` containment.
2. #29620 — share URI-native manifest path resolution.
3. **This PR** — keep selected plugin roots and resources URI-native.
4. #29626 — load executor skills without host path conversion.
5. #29628 — resolve executor MCP working directories without host path
conversion.

codex-rs/core/src/thread_manager_tests.rs

@@ -453,7 +453,7 @@ async fn start_thread_seeds_extension_data_for_mcp_and_lifecycle_contributors()
             id: id.to_string(),
             location: CapabilityRootLocation::Environment {
                 environment_id: environment_id.to_string(),
-                path: format!("/plugins/{id}"),
+                path: PathUri::parse(&format!("file:///plugins/{id}")).expect("plugin root URI"),
             },
         }]);
         init