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/utils/path-uri/src/lib.rs

@@ -246,10 +246,10 @@ impl PathUri {
     /// Returns true when this URI is lexically equal to or below `base`.
     ///
     /// Containment is computed using URI authority and path-segment boundaries,
-    /// without consulting the host filesystem. Percent-encoded path separators
-    /// fail closed because native path conversion may interpret them as segment
-    /// boundaries. Opaque fallback URIs created by [`Self::from_abs_path`] only
-    /// contain themselves.
+    /// without consulting the host filesystem. Percent-encoded native path
+    /// separators fail closed because native path conversion may interpret them
+    /// as segment boundaries. Opaque fallback URIs created by
+    /// [`Self::from_abs_path`] only contain themselves.
     pub fn starts_with(&self, base: &Self) -> bool {
         if self == base {
             return true;