feat: support Anthropic web search (server_tool_use) translation

[jackylam0812]

Problem

When using copilot-api with Claude Code or other Anthropic API clients that enable web search, the server-side tool types (server_tool_use, server_tool_result, web_search_tool_result) are not recognized. This causes:

1. translateAnthropicToolsToOpenAI to crash when encountering server tools (no input_schema)
2. handleUserMessage to silently drop server_tool_result / web_search_tool_result blocks
3. handleAssistantMessage to silently drop server_tool_use blocks
4. No web search capability at all through the proxy

Solution

Type definitions (anthropic-types.ts)

• Added AnthropicServerTool, AnthropicServerToolUseBlock, AnthropicServerToolResultBlock, AnthropicWebSearchToolResultBlock types
• Extended AnthropicUserContentBlock and AnthropicAssistantContentBlock unions

Message translation (non-stream-translation.ts)

• translateAnthropicToolsToOpenAI: Filter out server tools (they have no input_schema) instead of crashing
• handleUserMessage: Convert server_tool_result / web_search_tool_result blocks into user text messages with search context
• handleAssistantMessage: Convert server_tool_use blocks into text annotations
• Export hasWebSearchTool helper for handler use

Web search proxy (handler.ts)

• When web search server tools are present and mode is non-streaming:
  • Detect search intent from the model's response
  • Perform proxy search via a separate gpt-4o Copilot request
  • Return synthetic server_tool_use + web_search_tool_result blocks

Limitations

• Web search proxy currently works in non-streaming mode only
• Search is performed via gpt-4o as a proxy (not direct Bing API)
• Streaming support can be added in a follow-up PR

Testing

Tested with Claude Code + copilot-api v0.7.0:

• ✅ Web search requests properly translated and proxied
• ✅ Existing tool_use / tool_result flow unaffected
• ✅ Models without web search work as before