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/core/core.go

@@ -69,6 +69,11 @@ func NewFromConfig(cfg *config.Config) (*Core, error) {
 	if err := c.rebuildRegistry(); err != nil {
 		return nil, err
 	}
+	// Seed built-in preset templates on first run so the WebUI shows
+	// "从模板创建" for popular providers out of the box.
+	if err := c.seedPresetTemplates(); err != nil {
+		return nil, fmt.Errorf("seed preset templates: %w", err)
+	}
 	return c, nil
 }
 
@@ -162,6 +167,18 @@ func (c *Core) seedKeys() error {
 	return nil
 }
 
+// seedPresetTemplates adds the built-in provider presets to the template
+// store on first run. It only adds templates whose name is not already
+// present, so a user's edits to a preset survive restarts and a deliberately
+// deleted preset does not silently return on the next start (its name is
+// remembered in the store's preset marker list).
+func (c *Core) seedPresetTemplates() error {
+	if len(config.PresetTemplates) == 0 {
+		return nil
+	}
+	return c.store.SeedTemplates(config.PresetTemplates)
+}
+
 // saveConfig writes the current config (including auto rules and keys) back
 // to config.yaml.
 func (c *Core) saveConfig() error {