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.go

@@ -318,6 +318,7 @@ type RuntimeConfig struct {
 	SourceTemplates  []SourceTemplate  `json:"source_templates,omitempty"`
 	DeletedSources   []string          `json:"deleted_sources,omitempty"`
 	DeletedAdapters  []string          `json:"deleted_adapters,omitempty"`
+	PresetTemplates  []string          `json:"preset_templates,omitempty"` // preset names the user has seen (or deleted) — never re-seeded
 	Keys             []GWKey           `json:"keys,omitempty"`
 	Auto             []ModelScope      `json:"auto,omitempty"`
 	AutoImage        []ModelScope      `json:"auto_image,omitempty"`