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/codex_app_server_protocol.v2.schemas.json

@@ -15258,9 +15258,13 @@
       "$schema": "http://json-schema.org/draft-07/schema#",
       "properties": {
         "cwd": {
-          "type": [
-            "string",
-            "null"
+          "anyOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            },
+            {
+              "type": "null"
+            }
           ]
         },
         "mode": {
@@ -15322,4 +15326,4 @@
   },
   "title": "CodexAppServerProtocolV2",
   "type": "object"
-}
+}