Add skip_record_validation_for indexer config for sampled backfill validation

[vermatron]

Why

During large backfills of already-validated data, per-record JSON schema validation is wasted work. Every record walks the full schema (regex, enum, min/max, format, abstract-type discriminators) even though the source has already been validated upstream. Today there's no way to trade that cost for throughput.

This adds a config option that skips per-record validation for a chosen fraction of records, per GraphQL type, while keeping a sampled slice validated as a canary so schema drift still surfaces. It's a sibling of the existing skip_derived_indexing_type_updates backfill knob and follows the same shape.

Design notes:

• skip_record_validation_for maps a type name to a fraction in [0.0, 1.0]. 0.0 (or an absent key) validates everything, 1.0 skips everything, and values in between sample. The value is the fraction skipped, so 0.9 skips 90% and validates 10%.

• The event envelope is always validated. Only the per-record schema walk gets sampled.

• Skipping isn't silent. Processor counts skipped records per batch and logs one RecordValidationSkipped line with per-type counts, the same way it logs ElasticGraphIndexingLatencies. Logging per record would drown a 1.0 backfill in log lines, so the count is aggregated per batch.

• Safety net for skipped abstract-type records: once validation is off, a record with a missing or unknown __typename can reach RecordPreparer. It now raises a typed RecordPreparer::UnknownTypeError, and Factory#build rescues it into a FailedEventError so one bad record fails on its own instead of crashing the batch. The rescue is narrow (a typed error, not a bare rescue KeyError) so it can't hide schema-artifact bugs, and its message drops the offending value to avoid leaking record data. A pre-walk check was considered and dropped: it would re-implement prepare_for_index's recursion over nested fields and drift out of sync with it.

• The field defaults to {}, so nothing changes unless you set it. Additive and minor-release-safe.

What

Config:

  indexer:
    skip_record_validation_for:
      Widget: 0.9      # skip validation for 90% of Widget records, validate 10%
      Component: 1.0   # skip validation for all Component records

• config.rb: new skip_record_validation_for JSON schema property (object, per-type number in [0, 1] additionalProperties: false, default {}); convert_values coerces rates to Float.
• operation/factory.rb: new skip_validation?(type, event) helper; build skips record validation when sampled, records the skipped type on the result, and rescues UnknownTypeError. BuildResult gains validation_skipped_for.
• processor.rb: aggregate RecordValidationSkipped log per batch when any record was skipped.
• record_preparer.rb: new RecordPreparer::UnknownTypeError; the abstract-type fetch raises it with a value-free message.
• indexer.rb: wire config.skip_record_validation_for into the factory.
• RBS signatures updated for all of the above.
• elasticgraph-local config_schema.yaml: the new property added to the shared config validation schema.

Verification

• script/run_specs (COVERAGE=1, real Elasticsearch): 5222 examples, 0 failures. Coverage holds at the project's expected level (the one non-100% file, gem_spec.rb, is pre-existing and untouched here).
• script/type_check (Steep): no type errors.
• script/lint (Standard Ruby): 890 files, no offenses.
• script/spellcheck (codespell): clean.
• bundle exec rake schema_artifacts:check: up to date (runtime config only, no artifact changes).
• bundle exec rake site:validate: 148 runs, 0 failures.

New tests:

• config_spec.rb: integer YAML rates coerce to Float (1 to 1.0), and out-of-range rates (1.5, -0.1) are rejected at config load.
• operation/factory_spec.rb: a skipped type builds operations without record validation; non-skipped types still fail on bad records; envelope validation still runs for skipped types; fractional sampling (stubbed Zlib.crc32 for both branches); retry stability; the derived-index path under skip; and the unknown-__typename case returning a FailedEventError instead of raising.
• processor_spec.rb: a batch with skips logs one RecordValidationSkipped with the right count/counts_by_type; a batch with no skips logs none.

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