[BUG](fn-consumer): Use live backfill frontier

[tanujnay112]

Description of changes

During attached-function backfill, the executor processes the live record segment through the collection's current log position, but the batch was labeled with the earlier incremental fetch frontier. Idempotent functions therefore saved a frontier behind the records they had actually processed, allowing later executions to count overlapping records again.

• Derive the executor's pulled log offset from the existing is_for_backfill signal.
• Use the input collection's live log position for both the backfill batch frontier and the completion progress recorded after execution.
• Keep ordinary execution on the fetched pulled_log_offset.
• Rename the executor batch field from completion_offset to pulled_log_offset to reflect its actual meaning.

Test plan

• cargo test -p worker test_resolve_pulled_log_offset
• cargo test -p worker count_to_file_async::tests
• cargo clippy -p worker --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

Migration plan

No migration required. This changes an internal Rust field name and attached-function backfill frontier selection.

Observability plan

Existing attached-function completion logs expose the resulting completion frontier.

Documentation Changes

None; no user-facing API changes.

[claude]

Claude Code Review

This repository is configured for manual code reviews. Comment @claude review for a one-time review, or @claude review always to subscribe this PR to a review on every future push.

_{Tip: disable this comment in your organization's Code Review settings.}

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

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?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[tanujnay112]

• [BUG](fn-consumer): Use live backfill frontier #7423 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[rescrv]

I realize we should never hit this. It still feels wrong to just cast.