GitHub Actions updates and improvements for 7.1

[desrosj]

This ticket is for various updates and improvements for Core's GitHub Actions workflows.

Previously:

• 7.0 (#64227)
• 6.9 (#63170)
• 6.8 (#62221)
• 6.7 (#61564)
• 6.6 (#60733)
• 6.5 (#59805)
• 6.4 (#58867)
• 6.3 (#57865)
• 6.2 (#57572)

Because of the failure() check, the failure job runs even when the message's payload is not properly prepared.

This adds an additional check to guard against scenarios where the payload output is empty.

This was discovered in ​this workflow run. The raw logs are being attached so that they persist after the 90 day retention period for logs passes.

Trac ticket: Core-64893.

## Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Opus 4.6
Used for: Investigating and creating the initial PR.

This switches from Xdebug to PCOV for generating the unit test coverage report. PCOV should be substantially faster than Xdebug in coverage mode.

If the time savings are as they should be, it might be worth installing PCOV into the container images. This PR installs it prior to running the coverage tests.

Trac ticket: https://core.trac.wordpress.org/ticket/64893

## Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Opus 4.7
Used for: Initial planning for the switch to PCOV. Tweaked and verified by me.