app-server: require absolute cwd for windowsSandbox/setupStart (#13833)

## Summary
- require windowsSandbox/setupStart.cwd to be an AbsolutePathBuf
- reject relative cwd values at request parsing instead of normalizing
them later in the setup flow
- add RPC-layer coverage for relative cwd rejection and update the
checked-in protocol schemas/docs

## Why
windowsSandbox/setupStart was carrying the client-provided cwd as a raw
PathBuf for command_cwd while config derivation normalized the same
value into an absolute policy_cwd.

That left room for relative-path ambiguity in the setup path, especially
for inputs like cwd: "repo". Making the RPC accept only absolute paths
removes that split entirely: the handler now receives one
already-validated absolute path and uses it for both config derivation
and setup.

This keeps the trust model unchanged. Trusted clients could already
choose the session cwd; this change is only about making the setup RPC
reject relative paths so command_cwd and policy_cwd cannot diverge.

## Testing
- cargo test -p codex-app-server windows_sandbox_setup (run locally by
user)
- cargo test -p codex-app-server-protocol windows_sandbox (run locally
by user)

codex-rs/app-server-protocol/schema/json/v2/WindowsSandboxSetupStartParams.json

@@ -1,6 +1,10 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
   "definitions": {
+    "AbsolutePathBuf": {
+      "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
+      "type": "string"
+    },
     "WindowsSandboxSetupMode": {
       "enum": [
         "elevated",
@@ -11,9 +15,13 @@
   },
   "properties": {
     "cwd": {
-      "type": [
-        "string",
-        "null"
+      "anyOf": [
+        {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        },
+        {
+          "type": "null"
+        }
       ]
     },
     "mode": {
@@ -25,4 +33,4 @@
   ],
   "title": "WindowsSandboxSetupStartParams",
   "type": "object"
-}
+}