feat(v2): add ReasoningLevel option; thinking/reasoning across providers

Introduces an opt-in level-based reasoning toggle (low/medium/high) that
each provider translates to its native parameter:

- Anthropic: thinking.budget_tokens (1024/8000/24000), with temperature
  forced to default and MaxTokens auto-grown above the budget.
- OpenAI/xAI/Groq via openaicompat: reasoning_effort string, gated by a
  new Rules.SupportsReasoning predicate so non-reasoning models don't
  receive the parameter. xAI uses Rules.MapReasoningEffort to remap
  "medium" to "high" since its API only accepts low|high.
- Google: thinking_config.thinking_budget + include_thoughts:true.
- DeepSeek: SupportsReasoning=false (reasoner is always-on; the
  reasoning_content trace was already extracted via openaicompat).

Reasoning content is surfaced as Response.Thinking on Complete and as
StreamEventThinking deltas during streaming. Provider-side: extracted
from Anthropic thinking content blocks, Google's part.Thought=true
parts, and the non-standard reasoning_content field that DeepSeek and
Groq emit (parsed out of raw JSON since openai-go doesn't type it).

Public API:
  - llm.ReasoningLevel + ReasoningLow/Medium/High constants
  - llm.WithReasoning(level) request option
  - Model.WithReasoning(level) for baked-in defaults
  - provider.Request.Reasoning, provider.Response.Thinking
  - provider.StreamEventThinking

Tests cover Rules-based gating, MapReasoningEffort, reasoning_content
extraction (Complete + Stream), Anthropic budget mapping, and
temperature suppression when thinking is enabled. Existing behavior is
unchanged when Reasoning is the empty string.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

v2/xai/xai.go

@@ -25,5 +25,16 @@ func New(apiKey, baseURL string) *Provider {
 		SupportsVision: func(m string) bool {
 			return strings.Contains(m, "vision")
 		},
+		// Reasoning is supported on grok-3-mini and grok-4 family. The xAI
+		// API only accepts low|high (no medium); we map medium up to high.
+		SupportsReasoning: func(m string) bool {
+			return strings.Contains(m, "grok-3-mini") || strings.Contains(m, "grok-4")
+		},
+		MapReasoningEffort: func(level string) string {
+			if level == "medium" {
+				return "high"
+			}
+			return level
+		},
 	})
 }