fix: anthropic tool-call round-trip, cache zero-hit parity, round-robin load balancing

anthropic.lua v3.0.0:
- Issue 1: tool_result/tool_use round-trip
- Issue 3: thinking default OFF (opt-in via extra_body.thinking)
- Issue 4: tool_choice mapping
- Issue 5: collect_blocks preserves unknown part types
- message_stop no longer emits done=true (was overwriting tool_calls finish_reason)
- cache_read_input_tokens normalized even at 0

gemini.lua:
- transform_response was missing cachedContentTokenCount

openai.lua (Issue 6):
- transform_error handles flat envelopes, nginx HTML, bare text

chat.go mergeUsage:
- Keep PromptTokensDetails even when CachedTokens=0

scheduler.go:
- Remove sort.SliceStable by Pref; round-robin cursor is the only LB mechanism

provider.go ModelAvailable:
- Also check Pref() > prefMin, persistently failing slots exit cands

presets.go:
- 17 built-in source templates

Tests: 6 new test functions, 2 updated for new semantics

internal/config/config_test.go

@@ -318,3 +318,55 @@ sources:
 		t.Fatalf("after no-op: %v %v", len(cfg.Sources), err)
 	}
 }
+
+// TestSeedTemplatesOnce verifies preset templates are seeded exactly once:
+// a user's edit survives a restart, and a deliberately deleted preset does
+// not silently come back.
+func TestSeedTemplatesOnce(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "runtime.json")
+	presets := []SourceTemplate{
+		{Name: "DeepSeek", BaseURL: "https://api.deepseek.com", Adapter: "deepseek",
+			Models: []Model{{ID: "deepseek-chat", Kind: "chat"}}},
+		{Name: "OpenAI", BaseURL: "https://api.openai.com/v1", Adapter: "openai",
+			Models: []Model{{ID: "gpt-4o", Kind: "chat"}}},
+	}
+
+	// first run: both presets land in the store
+	s1 := NewStore(path)
+	if err := s1.Load(); err != nil {
+		t.Fatal(err)
+	}
+	if err := s1.SeedTemplates(presets); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	if got := len(s1.ListTemplates()); got != 2 {
+		t.Fatalf("after seed len = %d, want 2", got)
+	}
+
+	// user edits one preset, deletes the other
+	edited := presets[0]
+	edited.BaseURL = "https://my-proxy.internal/v1"
+	if err := s1.UpsertTemplate(edited); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := s1.RemoveTemplate("OpenAI"); err != nil {
+		t.Fatal(err)
+	}
+
+	// restart: re-seeding must be a no-op for both
+	s2 := NewStore(path)
+	if err := s2.Load(); err != nil {
+		t.Fatal(err)
+	}
+	if err := s2.SeedTemplates(presets); err != nil {
+		t.Fatalf("reseed: %v", err)
+	}
+	tpls := s2.ListTemplates()
+	if len(tpls) != 1 {
+		t.Fatalf("after restart len = %d, want 1 (deleted preset must not return): %+v", len(tpls), tpls)
+	}
+	if tpls[0].Name != "DeepSeek" || tpls[0].BaseURL != "https://my-proxy.internal/v1" {
+		t.Fatalf("user edit lost on restart: %+v", tpls[0])
+	}
+}