fix: stop coverage without start

[aaltat]

Fixes: #3991

[Copilot]

Pull request overview

This PR addresses issue #3991 by ensuring Stop Coverage behaves correctly (and logs accurately) when coverage was never started for the active page, avoiding misleading “report generated” behavior.

Changes:

• Node side returns a sentinel body (ROBOT_FRAMEWORK_BROWSER_NO_SET) when stopping coverage without a prior start.
• Python keyword stop_coverage detects that sentinel, logs that no report was generated, and returns None.
• Adds an acceptance test covering the “only stop coverage” scenario and verifying the expected logs and return value.

Reviewed changes

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

File	Description
node/playwright-wrapper/playwright-state.ts	Returns a sentinel value instead of an empty string when coverage was not started.
Browser/keywords/coverage.py	Interprets the sentinel, logs “no report generated”, and returns None.
atest/test/01_Browser_Management/coverage.robot	Adds an acceptance test validating logs and ${None} return when only Stop Coverage is called.