Move record id preparation into the JSON ingestion adapter

[myronmarston]

The indexer copied the envelope `id` into the record for every ingestion format. Only the JSON adapter knows `record` is a Hash, so the copy moves there — this drops the last format-specific `is_a?` check from the operation factory and prepares for a typed `Event` whose `record` type is a per-format generic parameter.

• `ValidationResult` now carries the event to build operations from, so an adapter can return a prepared copy of the event.
• `Operation::Factory` narrows `validation_result.record_preparer`/`.event` with `# : ...` type comments instead of `or raise`: both are always present together by construction once `failure` is nil, so the assertion can never fire — this states the invariant to Steep rather than guarding against an unreachable case at runtime.

🤖 Generated with Claude Code

[myronmarston]

This change is part of the following stack:

• Point test datastore URLs at 127.0.0.1 instead of localhost #1400
  • Move record id preparation into the JSON ingestion adapter #1393 ◀
    • Report indexing failures through a _IndexingFailure interface #1395
      • Make _RecordPreparer generic #1396
        • Validate the event envelope before building operations #1397
          • Route events through a typed Event value #1398
    • Route indexing events through a typed Event value end to end #1394

_{Change managed by git-spice.}