feat(providers): explicit prompt cache for DashScope/Qwen (#1127)

* feat(providers): explicit prompt cache for DashScope/Qwen

Extend OpenAI-compat path with Anthropic-style cache_control:ephemeral
inline blocks for Alibaba DashScope endpoints (provider_type=bailian or
URL contains "dashscope"). Verified live: qwen3.6-plus / qwen3.5-plus /
qwen3-coder-plus all return 99.7% cache hit rate on 6K-token prefix,
yielding ~89% token cost reduction on cached prefix (90% Alibaba discount).

- isDashScope(): 3-source detection (URL + providerType + name) handles
  reverse-proxied endpoints; covers both "dashscope" and "bailian"
- buildRequestBody wraps system content with SplitSystemPromptForCache
  (exported from anthropic_request.go) using <!-- GOCLAW_CACHE_BOUNDARY -->
- Tool prefix cache: cache_control on last tool definition, with 4-marker
  budget guard
- Parse cache_creation_input_tokens from prompt_tokens_details into
  Usage.CacheCreationTokens; propagates via existing span metadata writers
- Runtime escape hatch: GOCLAW_DISABLE_DASHSCOPE_CACHE=true
- Live integration smoke test (build tag integration, env-gated)

* chore(providers): polish per PR #1127 review

- Use strings.Repeat instead of custom repeat() helper in smoke test
- Clarify BuildRequestBodyForTest is test-only, not public API

* feat(providers): enable thinking for Qwen 3.7/3.6, observe cache in smoke test

Qwen3.7-plus and qwen3.6-plus support deep thinking but were missing from
dashscopeThinkingModels, so enable_thinking/thinking_budget was silently
skipped. Add both to the whitelist and test.

Add qwen3.7-plus to the cache smoke test as cache-optional: Alibaba's
context-cache doc does not yet list 3.7/3.6 for explicit cache, and the
cache wrap is a safe no-op when unsupported, so a no-cache result is logged
rather than failed to avoid a flaky live assertion.

Claude-Session: https://claude.ai/code/session_01X3jkrc7N3ar8ZGzUJWyNS5

* fix(providers): preserve DashScope detection for proxy routes

cmd/gateway_providers.go

@@ -101,7 +101,8 @@ func registerProviders(registry *providers.Registry, cfg *config.Config, modelRe
 		if base == "" {
 			base = "https://coding-intl.dashscope.aliyuncs.com/v1"
 		}
-		registry.Register(providers.NewOpenAIProvider("bailian", cfg.Providers.Bailian.APIKey, base, "qwen3.5-plus"))
+		registry.Register(providers.NewOpenAIProvider("bailian", cfg.Providers.Bailian.APIKey, base, "qwen3.5-plus").
+			WithProviderType(store.ProviderBailian))
 		slog.Info("registered provider", "name", "bailian")
 	}
 
@@ -356,7 +357,8 @@ func registerProvidersFromDB(registry *providers.Registry, provStore store.Provi
 			if base == "" {
 				base = "https://coding-intl.dashscope.aliyuncs.com/v1"
 			}
-			registry.RegisterForTenant(p.TenantID, providers.NewOpenAIProvider(p.Name, p.APIKey, base, "qwen3.5-plus"))
+			registry.RegisterForTenant(p.TenantID, providers.NewOpenAIProvider(p.Name, p.APIKey, base, "qwen3.5-plus").
+				WithProviderType(p.ProviderType))
 		case store.ProviderZai:
 			base := p.APIBase
 			if base == "" {