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/scheduler/scheduler.go

@@ -248,10 +248,9 @@ func (s *Scheduler) chainDrive(ctx context.Context, chain *Chain, req *types.Cha
 			ce.Skipped = append(ce.Skipped, fmt.Sprintf("tier %d: no schedulable slot (cooling or quota exhausted)", tn.Tier))
 			continue
 		}
-		// preference orders a same-tier run; stable so equal prefs keep order
-		sort.SliceStable(cands, func(i, j int) bool {
-			return cands[i].Prov.Pref(cands[i].Model) > cands[j].Prov.Pref(cands[j].Model)
-		})
+		// No Pref sort: load balancing is done by round-robin cursor.
+		// Persistently failing slots are excluded by ModelAvailable
+		// (which checks Pref > prefMin).
 		base := tn.NextStart()
 		res := runTier(ctx, tn, cands, base, req, stream)
 		if res.resp != nil || res.chunks != nil {
@@ -350,9 +349,7 @@ func (s *Scheduler) ChainImage(ctx context.Context, chain *Chain, req *types.Ima
 			ce.Skipped = append(ce.Skipped, fmt.Sprintf("tier %d: no schedulable slot (cooling)", tn.Tier))
 			continue
 		}
-		sort.SliceStable(cands, func(i, j int) bool {
-			return cands[i].Prov.Pref(cands[i].Model) > cands[j].Prov.Pref(cands[j].Model)
-		})
+		// No Pref sort: load balancing is done by round-robin cursor.
 		base := tn.NextStart()
 		var hard []TierError
 		for i := 0; i < len(cands); i++ {