[BUG](frontend): Meter IDs-only deletes

[Jokasa7]

Description of changes

• Improvements & Bug fixes
  • Close the CollectionReadContext after the delete selector branches have
    converged, instead of closing it only after a where lookup.
  • Submit the existing CollectionRead(GetForDelete) event for IDs-only and
    empty-selector delete paths as well as where-based deletes.
  • Preserve the query-size metadata collected by the where branch and the
    existing metering_events_sent.read success counter.
  • Add an integration regression test that exercises an IDs-only delete under
    a real metering context and verifies that exactly one read event is emitted
    with the expected action and region.
• New functionality
  • None. This corrects the billing-event lifecycle described in [BUG] IDs-only delete silently drops CollectionReadContext metering event #6695 without
    changing the delete API or record-selection behavior.

Closes #6695.

Test plan

• Tests pass locally with cargo test for Rust.
  • Before the production change, the new focused test failed with zero
    CollectionRead events where one was expected.
  • cargo test -p chroma-frontend --test test_delete_limit test_delete_by_ids_emits_collection_read_metering_event -- --exact --nocapture passes after the fix.
  • cargo test -p chroma-frontend --test test_delete_limit -- --nocapture
    passes all 8 tests.
  • cargo clippy -p chroma-frontend --test test_delete_limit -- -D warnings
    passes.
  • cargo fmt --all --check and git diff --check pass.

Migration plan

No migration or compatibility step is required. The change only ensures that
an already-created read metering context reaches the existing event receiver on
every delete-selector path.

Observability plan

No new metric is introduced. Successful submission continues to increment the
existing metering_events_sent.read counter, and the emitted event retains the
existing GetForDelete action and region fields. That counter and the billing
event stream can be used to confirm that IDs-only deletes are no longer absent.

Documentation Changes

No user-facing API, configuration, or behavior documentation changes are
needed. This is an internal metering correctness fix with regression coverage.

[claude]

Claude Code Review

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[github-actions]

Reviewer Checklist

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
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Quality

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