fix(gateway): honor Anthropic protocol base URLs.

[wucm667]

问题

Adaptive accounts with different Anthropic and OpenAI-compatible upstream bases ignored api_base_urls.anthropic on the native Anthropic gateway and count_tokens paths, sending requests to the wrong upstream endpoint.

根因

These GatewayService request builders used GetBaseURL() directly, unlike the corresponding protocol-aware OpenAI gateway path. That also made Ollama Cloud capability clamping and authentication-header mode checks use the wrong base.

改动

• Resolve the Anthropic gateway base from GetAnthropicProtocolBaseURL() with a fallback to GetBaseURL().
• Apply the selected base consistently to native Messages, API-key passthrough, count_tokens, Ollama Cloud max_tokens clamping, and authentication-header selection.
• Add regression coverage for adaptive base selection, provider fallback, ordinary Anthropic API-key compatibility, count_tokens, and Ollama Cloud behavior.

验证

• 已验证：定向 GatewayService 回归测试通过。
• 已验证：go vet -tags=unit ./internal/service 通过。
• 已验证：go test ./internal/service -count=1 通过。
• 已验证：go test -tags=unit ./internal/service -count=1 通过。
• 已验证：gofmt 与 git diff --check 通过。

Fixes #8074