feat: add extended thinking support for Anthropic models (#1076)

* feat: add extended thinking support for Anthropic models

Support configurable thinking levels (off/low/medium/high/xhigh/adaptive)
via `agents.defaults.thinking_level` config field.

- "adaptive": uses Anthropic's adaptive thinking API (Claude 4.6+)
- "low/medium/high/xhigh": uses budget_tokens (all thinking-capable models)
- "off": disables thinking (default)

API constraints handled:
- Temperature cleared when thinking is enabled
- budget_tokens clamped to max_tokens-1
- Thinking response blocks parsed into Reasoning field

Relates to #645, #966

* fix: address PR review feedback for thinking support

- Add ThinkingCapable interface for provider capability detection
- Warn when thinking_level is set but provider doesn't support it
- Warn when temperature is cleared due to thinking enabled
- Adjust budget values per Anthropic best practices (medium=16K, xhigh=64K)
- Add budget clamp warning and 80% threshold warning
- Add parseResponse thinking block tests
- Add thinking_level field to config.example.json

* refactor: move ThinkingLevel from AgentDefaults to ModelConfig

Thinking is a model-level capability, not a global agent property.
Per-model config avoids silent ignoring on non-Anthropic providers
and eliminates spurious warning logs in multi-provider setups.

Addresses PR #1076 review feedback from @yinwm.

config/config.example.json

@@ -22,7 +22,8 @@
       "model_name": "claude-sonnet-4.6",
       "model": "anthropic/claude-sonnet-4.6",
       "api_key": "sk-ant-your-key",
-      "api_base": "https://api.anthropic.com/v1"
+      "api_base": "https://api.anthropic.com/v1",
+      "thinking_level": "high"
     },
     {
       "model_name": "gemini",