Treat missing JSON paths as expected failures instead of errors

[cmttt]

Summary

• Remove MissingJsonPath exception class from json_utils.py
• Missing JSON paths now raise ExpectedUdfException instead, which is already filtered from error_infos before serialization to BigTable
• Remove MissingJsonPath from _is_spammy_exception in executor (no longer needed)
• Update test_json_data to expect no error_infos for missing paths with required=True

This eliminates noisy error traces for the common case where actions only have a subset of JSON fields (~498 JsonData calls across ~6000 rules). InvalidJsonType still raises for real type mismatches.

Test plan

• Existing test_json_data tests pass with updated assertions
• InvalidJsonType still raised for real type mismatches
• Deploy to staging, verify reduced Sentry noise and smaller BigTable payloads