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/anthropic/thinking_test.go

@@ -0,0 +1,83 @@
+package anthropic
+
+import (
+	"testing"
+
+	"gitea.stevedudenhoeffer.com/steve/go-llm/v2/provider"
+
+	anth "github.com/liushuangls/go-anthropic/v2"
+)
+
+func TestBuildRequest_ThinkingByLevel(t *testing.T) {
+	p := New("k")
+	cases := []struct {
+		level     string
+		wantBudget int
+	}{
+		{"", 0},
+		{"low", thinkingBudgetLow},
+		{"medium", thinkingBudgetMedium},
+		{"high", thinkingBudgetHigh},
+	}
+	for _, tc := range cases {
+		t.Run("level="+tc.level, func(t *testing.T) {
+			req := provider.Request{
+				Model:    "claude-opus-4-7",
+				Reasoning: tc.level,
+				Messages: []provider.Message{{Role: "user", Content: "hi"}},
+			}
+			out := p.buildRequest(req)
+			if tc.wantBudget == 0 {
+				if out.Thinking != nil {
+					t.Fatalf("Thinking should be nil for level=%q, got %+v", tc.level, out.Thinking)
+				}
+				return
+			}
+			if out.Thinking == nil {
+				t.Fatalf("Thinking should be set for level=%q", tc.level)
+			}
+			if out.Thinking.Type != anth.ThinkingTypeEnabled {
+				t.Errorf("Thinking.Type = %q, want enabled", out.Thinking.Type)
+			}
+			if out.Thinking.BudgetTokens != tc.wantBudget {
+				t.Errorf("BudgetTokens = %d, want %d", out.Thinking.BudgetTokens, tc.wantBudget)
+			}
+			if out.MaxTokens <= tc.wantBudget {
+				t.Errorf("MaxTokens (%d) must exceed BudgetTokens (%d)", out.MaxTokens, tc.wantBudget)
+			}
+		})
+	}
+}
+
+func TestBuildRequest_ThinkingDropsTemperature(t *testing.T) {
+	p := New("k")
+	temp := 0.7
+	req := provider.Request{
+		Model:       "claude-opus-4-7",
+		Reasoning:   "high",
+		Temperature: &temp,
+		Messages:    []provider.Message{{Role: "user", Content: "hi"}},
+	}
+	out := p.buildRequest(req)
+	if out.Temperature != nil {
+		t.Errorf("Temperature should be dropped when thinking is enabled, got %v", *out.Temperature)
+	}
+}
+
+func TestBuildRequest_NoThinkingPreservesTemperature(t *testing.T) {
+	p := New("k")
+	temp := 0.7
+	req := provider.Request{
+		Model:       "claude-opus-4-7",
+		Temperature: &temp,
+		Messages:    []provider.Message{{Role: "user", Content: "hi"}},
+	}
+	out := p.buildRequest(req)
+	if out.Temperature == nil {
+		t.Fatal("Temperature should be set when thinking is disabled")
+	}
+	got := float64(*out.Temperature)
+	if got < 0.69 || got > 0.71 {
+		t.Errorf("Temperature should be ~0.7 when thinking is disabled, got %v", got)
+	}
+}