Populate workspace kind on Codex turn events (#25135)

## Summary
- carry `workspace_kind` from Responses API client metadata into the
turn resolved analytics fact
- serialize the optional value on `codex_turn_event`
- cover both the turn metadata source and turn event serialization

The `workspace_kind` tells us whether a thread had a project attached vs
projectless. this is an indicator for who is adopting Codex for
knowledge work outside of coding

## Testing
- `env UV_CACHE_DIR=/private/tmp/uv-cache
/private/tmp/cargo-tools/bin/just fmt`
- `env PATH=/private/tmp/cargo-tools/bin:$PATH
CARGO_HOME=/private/tmp/cargo-home UV_CACHE_DIR=/private/tmp/uv-cache
/private/tmp/cargo-tools/bin/just test -p codex-analytics`
- `env PATH=/private/tmp/cargo-tools/bin:$PATH
CARGO_HOME=/private/tmp/cargo-home UV_CACHE_DIR=/private/tmp/uv-cache
/private/tmp/cargo-tools/bin/just test -p codex-core turn_metadata`

Paired with openai/openai#970661, which keeps forwarding the same
metadata key through Responses API headers.

codex-rs/core/src/session/turn.rs

@@ -640,6 +640,7 @@ async fn track_turn_resolved_config_analytics(
             sandbox_network_access: turn_context.network_sandbox_policy().is_enabled(),
             collaboration_mode: turn_context.collaboration_mode.mode,
             personality: turn_context.personality,
+            workspace_kind: turn_context.turn_metadata_state.workspace_kind(),
             is_first_turn,
         });
 }