Add explicit app and account targets to MCP resource reads (#47248)

## What changed

Allow `mcpServer/resource/read` to specify `target: {connectorId, linkId}` for direct hosted app reads without tool discovery, when supported by the backend. Require an explicit `linkId`: a string selects an account, while `null` requests no-auth access subject to backend policy.

Restrict direct targets to `codex_apps` and reject empty connector IDs, empty link IDs, and synthetic link IDs. Preserve originating app/account scope when `originCallId` and `threadId` are supplied, and retain discovery when `target` is omitted.

Update protocol schemas and TypeScript and Python models, preserving required-but-nullable `linkId` semantics in Python code generation.

## Testing

Extend resource-read tests to check target metadata with and without a thread, including explicit no-auth targets and omitted targets. Add Python model tests for string and null `linkId` values and rejection of a missing `linkId`.

GitOrigin-RevId: 8cf5595fbdd937193aeb529901fc95299f6e18ad

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -2046,6 +2046,17 @@
         "server": {
           "type": "string"
         },
+        "target": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/McpResourceReadTarget"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "description": "Explicit hosted app/account. Omit to retain legacy resource discovery."
+        },
         "threadId": {
           "type": [
             "string",
@@ -2062,6 +2073,25 @@
       ],
       "type": "object"
     },
+    "McpResourceReadTarget": {
+      "properties": {
+        "connectorId": {
+          "type": "string"
+        },
+        "linkId": {
+          "description": "Null explicitly requests no-auth access, subject to the app's resource policy.",
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "connectorId",
+        "linkId"
+      ],
+      "type": "object"
+    },
     "McpServerMigration": {
       "properties": {
         "name": {