feat(api): add tuning params (temperature, top_p, penalties, stop) to MessageRequest

MessageRequest was missing standard OpenAI-compatible generation tuning
parameters. Callers had no way to control temperature, top_p,
frequency_penalty, presence_penalty, or stop sequences.

Changes:
- Added 5 optional fields to MessageRequest (all Option, None by default)
- Wired into build_chat_completion_request: only included in payload when set
- All existing construction sites updated with ..Default::default()
- MessageRequest now derives Default for ergonomic partial construction

Tests added:
- tuning_params_included_in_payload_when_set: all 5 params flow into JSON
- tuning_params_omitted_from_payload_when_none: absent params stay absent

83 api lib tests passing, 0 failing.
cargo check --workspace: 0 warnings.

rust/crates/tools/src/lib.rs

@@ -3841,6 +3841,7 @@ impl ApiClient for ProviderRuntimeClient {
                 tools: (!tools.is_empty()).then(|| tools.clone()),
                 tool_choice: tool_choice.clone(),
                 stream: true,
+                ..Default::default()
             };
 
             let attempt = runtime.block_on(stream_with_provider(&entry.client, &message_request));