refactor(ai-proxy): add ServerRemoteTool and ServerRemoteToolBuilder

[Scra3]

Summary

• Add ServerRemoteTool for server-side tools (sourceType: 'server')
• Extract tool building logic into ServerRemoteToolBuilder
• Update RemoteTools to use ServerRemoteToolBuilder
• Update router types

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

Depends on

• #1417

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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