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/analytics/src/events.rs

@@ -794,6 +794,7 @@ pub(crate) struct CodexTurnEventParams {
     pub(crate) sandbox_network_access: bool,
     pub(crate) collaboration_mode: Option<&'static str>,
     pub(crate) personality: Option<String>,
+    pub(crate) workspace_kind: Option<String>,
     pub(crate) num_input_images: usize,
     pub(crate) is_first_turn: bool,
     pub(crate) status: Option<TurnStatus>,