Send installation id in Responses headers

[jiamingz42]

Summary

• Move x-codex-installation-id into the shared Responses identity headers so regular /responses HTTP and Responses websocket handshakes carry the same installation id header as /responses/compact.
• Keep existing client_metadata propagation intact.
• Skip the header when the installation id is empty or invalid, so clients do not send empty installation id headers.

Testing

• cargo test -p codex-core installation_id -- --nocapture
• cargo test -p codex-core --test responses_headers responses_stream_includes_subagent_header_on_review -- --nocapture
• cargo test -p codex-core --test all responses_websocket_streams_request -- --nocapture
• cargo test -p codex-core --test all remote_compact_replaces_history_for_followups -- --nocapture
• cargo fmt -- --config imports_granularity=Item --check
• cargo clippy -p codex-core --lib --no-deps -- -D warnings

Not run to completion:

• cargo clippy -p codex-core --tests -- -D warnings stopped on an existing unrelated clippy::while_let_loop warning in code-mode/src/runtime/mod.rs.