feat(agents): support thinkingDefault: "adaptive" for Anthropic models (#31227)

* feat(agents): support `thinkingDefault: "adaptive"` for Anthropic models

Anthropic's Opus 4.6 and Sonnet 4.6 support adaptive thinking where the
model dynamically decides when and how much to think.  This is now
Anthropic's recommended mode and `budget_tokens` is deprecated on these
models.

Add "adaptive" as a valid thinking level:
- Config: `agents.defaults.thinkingDefault: "adaptive"`
- CLI: `/think adaptive` or `/think auto`
- Pi SDK mapping: "adaptive" → "medium" effort at the pi-agent-core
  layer, which the Anthropic provider translates to
  `thinking.type: "adaptive"` with `output_config.effort: "medium"`
- Provider fallbacks: OpenRouter and Google map "adaptive" to their
  respective "medium" equivalents

Closes #30880

Made-with: Cursor

* style(changelog): format changelog with oxfmt

* test(types): fix strict typing in runtime/plugin-context tests

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/agents/pi-embedded-runner/extra-params.ts

@@ -518,6 +518,9 @@ function mapThinkingLevelToOpenRouterReasoningEffort(
   if (thinkingLevel === "off") {
     return "none";
   }
+  if (thinkingLevel === "adaptive") {
+    return "medium";
+  }
   return thinkingLevel;
 }
 
@@ -631,6 +634,7 @@ function mapThinkLevelToGoogleThinkingLevel(
     case "low":
       return "LOW";
     case "medium":
+    case "adaptive":
       return "MEDIUM";
     case "high":
     case "xhigh":

src/agents/pi-embedded-runner/utils.ts

@@ -6,6 +6,13 @@ export function mapThinkingLevel(level?: ThinkLevel): ThinkingLevel {
   if (!level) {
     return "off";
   }
+  // "adaptive" maps to "medium" at the pi-agent-core layer.  The Pi SDK
+  // provider then translates this to `thinking.type: "adaptive"` with
+  // `output_config.effort: "medium"` for models that support it (Opus 4.6,
+  // Sonnet 4.6).
+  if (level === "adaptive") {
+    return "medium";
+  }
   return level;
 }