feat(usage): surface Gemini thinkingConfig as reasoning effort

Usage records show a reasoning badge next to the model name for OpenAI and
Claude requests, but Gemini requests never got one. The extraction only read
the OpenAI/Claude shapes (`reasoning_effort`, `reasoning.effort`,
`output_config.effort`), while Gemini states its reasoning depth inside
`generationConfig.thinkingConfig` — so nothing was written to the usage
metadata and the list and detail views had no badge to render.

Read the Gemini shape too, as a fallback after the existing three so the
OpenAI and Claude paths are untouched:

- `thinkingLevel` / `thinking_level` wins when present, trimmed and
  lowercased, with the protobuf enum prefix stripped so `THINKING_LEVEL_HIGH`
  resolves like `high`.
- Otherwise `thinkingBudget` / `thinking_budget` goes through the existing
  shared budget ladder, yielding the same `low|medium|high|xhigh` vocabulary
  the badge already understands.
- Both camelCase and snake_case spellings are read, so a captured client body
  and a converted provider body resolve to the same label.
- `includeThoughts` alone is a visibility flag, not a depth, and produces no
  badge.

Two cases are handled explicitly rather than through the shared ladder:

- `thinkingBudget: 0` disables reasoning outright. The shared ladder maps
  `0..=1664` to `low`, which would report an explicitly disabled request as a
  shallow one, so it reports `none` instead.
- `THINKING_LEVEL_UNSPECIFIED` is the enum's "no explicit level" member, not a
  depth; it is rejected rather than surfaced as an `unspecified` badge.

The frontend needs no change: `UsageModelDisplay` already renders the badge
whenever the fields are present, and keeps the `high -> xhigh` mapping format
when the requested and upstream efforts differ.

crates/aether-usage/runtime/src/request_metadata.rs

@@ -663,6 +663,39 @@ mod tests {
         assert!(cleared.get("requested_reasoning_effort").is_none());
     }
 
+    #[test]
+    fn gemini_thinking_config_is_derived_into_client_and_provider_reasoning_metadata() {
+        let client_body = json!({
+            "generationConfig": {
+                "thinkingConfig": { "includeThoughts": true, "thinkingLevel": "HIGH" }
+            }
+        });
+        let provider_body = json!({
+            "generation_config": {
+                "thinking_config": { "thinking_budget": 8192 }
+            }
+        });
+
+        let metadata = attach_client_request_body_metadata(
+            Some(json!({ "trace_id": "trace-1" })),
+            Some(&client_body),
+        )
+        .expect("metadata should remain");
+        assert_eq!(metadata["requested_reasoning_effort"], "high");
+
+        let metadata = attach_provider_request_body_metadata(
+            Some(metadata),
+            Some("gemini:generate_content"),
+            Some("gemini-3.8-flash"),
+            Some("gemini-3.8-flash"),
+            Some(&provider_body),
+        )
+        .expect("metadata should remain");
+
+        assert_eq!(metadata["requested_reasoning_effort"], "high");
+        assert_eq!(metadata["provider_reasoning_effort"], "xhigh");
+    }
+
     #[test]
     fn provider_request_body_metadata_uses_final_provider_body_as_source_of_truth() {
         let metadata = Some(json!({