feat: add Claude API bridge - transparent OpenAI-to-Anthropic protocol conversion

When provider is set to "claude" in config, all OpenAI-compatible API calls
are automatically bridged to Anthropic Claude Messages API, including:

- Non-streaming and streaming chat completions
- Tool calls (function calling) with full bidirectional conversion
- Eino multi-agent via HTTP transport hook (claudeRoundTripper)
- System message extraction, auth header conversion (Bearer → x-api-key)
- SSE stream format conversion (content_block_delta → OpenAI delta)
- TestOpenAI handler support for Claude connectivity testing

Zero impact when provider is "openai" or empty (default behavior unchanged).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -129,6 +129,7 @@ type MCPConfig struct {
 }
 
 type OpenAIConfig struct {
+	Provider       string `yaml:"provider,omitempty" json:"provider,omitempty"` // API 提供商: "openai"(默认) 或 "claude"，claude 时自动桥接为 Anthropic Messages API
 	APIKey         string `yaml:"api_key" json:"api_key"`
 	BaseURL        string `yaml:"base_url" json:"base_url"`
 	Model          string `yaml:"model" json:"model"`