fix(events): type pull_executor concretely in _hold_leases (#1596)

[shobhitagnihotri69]

Summary

Types pull_executor concretely as concurrent.futures.ThreadPoolExecutor in _hold_leases instead of Any.

Related Issue

Closes #1596

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Documentation update
• Refactor / internal cleanup
• Other:

How Has This Been Tested?

• Verified with Python 3.12 compilation (py_compile).
• Verified with ruff check common/events/pubsub.py and ruff format --check common/events/pubsub.py (all checks passed).
• Caller _pull_loop statically establishes pull_executor from self._pull_executor (concurrent.futures.ThreadPoolExecutor | None) after narrowing away None. Because concurrent.futures is already imported at module scope (L49), this restores the concrete type without introducing any new imports or behavior changes.
• Preserved sibling subscriber: Any untouched as described in Type pull_executor concretely in _hold_leases instead of Any #1596 to maintain lazy import ergonomics for non-GCP environments.

Checklist

• I have read CONTRIBUTING.md
• I have added tests that cover my changes (or explained why none are needed)
• ruff check and ruff format --check pass
• mypy passes
• pytest passes locally
• I have updated relevant documentation (README, docs, etc.)
• I have updated CHANGELOG.md under the Unreleased section (if user-facing)

Additional Notes

Maintains byte-compatibility for downstream reconciliation and enterprise vendoring as noted in #1596 by @Eldad-Caura.