refactor(ai-proxy): introduce RemoteTool class hierarchy

[Scra3]

Summary

• Make RemoteTool an abstract base class with sourceType as a property
• Add McpServerRemoteTool for MCP server tools (sourceType: 'mcp-server')
• Add ServerRemoteTool for server-side tools (sourceType: 'server')
• Extract tool building logic into ServerRemoteToolBuilder for better extensibility
• Update tests to use concrete classes

This refactoring prepares the codebase for adding more server-side tools (slack, infogreffe, etc.) by providing a cleaner architecture.

Test plan

• [x] All unit tests pass (yarn workspace @forestadmin/ai-proxy test)
• [x] Lint passes (yarn workspace @forestadmin/ai-proxy lint)
• [x] Build passes (yarn workspace @forestadmin/ai-proxy build)

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