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/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -7042,9 +7042,10 @@
               "environmentId": {
                 "type": "string"
               },
-              "path": {
-                "type": "string"
-              },
+                "path": {
+                  "description": "Absolute path for the root in the selected environment.",
+                  "type": "string"
+                },
               "type": {
                 "enum": [
                   "environment"
@@ -20689,4 +20690,4 @@
   },
   "title": "CodexAppServerProtocol",
   "type": "object"
-}
+}