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/src/codex_message_processor.rs

@@ -6653,7 +6653,10 @@ impl CodexMessageProcessor {
         let config = Arc::clone(&self.config);
         let cli_overrides = self.cli_overrides.clone();
         let cloud_requirements = self.current_cloud_requirements();
-        let command_cwd = params.cwd.unwrap_or_else(|| config.cwd.clone());
+        let command_cwd = params
+            .cwd
+            .map(PathBuf::from)
+            .unwrap_or_else(|| config.cwd.clone());
         let outgoing = Arc::clone(&self.outgoing);
         let connection_id = request_id.connection_id;