FakeService: filter: Fix field-existance check

[Marenz]

This PR fixes a bug in the FakeService filter implementation where field existence was not properly checked before accessing optional protobuf fields. The fix replaces the walrus operator with explicit HasField() checks to correctly determine if optional fields are set.

[Copilot]

Pull Request Overview

This PR fixes a bug in the FakeService filter implementation where field existence was not properly checked before accessing optional protobuf fields. The fix replaces the walrus operator with explicit HasField() checks to correctly determine if optional fields are set.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/frequenz/client/dispatch/test/_service.py	Updates filter logic to use HasField() instead of walrus operator for checking protobuf field existence
tests/test_service.py	Adds comprehensive test coverage for start_time and end_time filtering functionality
RELEASE_NOTES.md	Documents the bug fix in release notes

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[Copilot]

The condition check order has changed from the original logic. The original code checked dispatch.duration before the time comparison, but the parentheses now group the time comparison separately. Consider moving dispatch.duration check outside the parentheses to maintain the original short-circuit behavior: if dispatch.duration and dispatch.start_time + dispatch.duration < _to_dt(end_from):

Suggested change

	if dispatch.duration and (
	dispatch.start_time + dispatch.duration < _to_dt(end_from)
	):
	if dispatch.duration and dispatch.start_time + dispatch.duration < _to_dt(end_from):