app-server: require absolute cwd for windowsSandbox/setupStart

[iceweasel-oai]

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)