fix(rpc-agent): return JSON error responses instead of HTML for exceptions

[slimee]

Summary

• Fix exception handling in forest_admin_rpc_agent package to return JSON error responses instead of HTML
• Add rescue StandardError block in register_rails method to catch exceptions
• Add handle_rails_exception method that uses ErrorTranslator to format errors as JSON
• Add test to verify 404 errors return proper JSON response with correct headers

Problem

When exceptions are raised in RPC agent routes (like NotFoundError for non-existent collections or resources), they were not caught and Rails would return HTML error pages instead of JSON.

Solution

Similar to how forest_admin_rails handles exceptions in its ForestController, the RPC agent now:

1. Catches all StandardError exceptions in the route handler
2. Uses ErrorTranslator to convert them to HTTP exceptions
3. Returns properly formatted JSON responses with:
   • Correct HTTP status code (e.g., 404)
   • Content-Type: application/json header
   • Custom headers (e.g., x-error-type: object-not-found)
   • Error details in standardized format

Test plan

• Add test for NotFoundError returning JSON 404 response
• Run existing test suite to ensure no regressions
• Manually test with a Rails app using RPC agent

🤖 Generated with Claude Code

[qltysh]

All good ✅
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