[flutter-gold] Consider neutral checks completed

[Piinks]

Fixes flutter/flutter#185832

This PR improves how Flutter Gold status updates are computed by accounting for completed checks that end in a NEUTRAL conclusion (alongside SUCCESS). It also streamlines logging and fortifies the check status evaluation.

• Updates the evaluation logic so that PR checks ending in NEUTRAL are no longer grouped alongside incomplete or failing tests.
• Added explicit status verification for check runs. If a check run does not have the status 'COMPLETED', it is now safely and reliably classified as incomplete.
• Moved the log.debug statement to only capture check runs that are truly incomplete or failing, significantly reducing log output noise.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[auto-submit]

auto label is removed for flutter/cocoon/5036, Failed to merge flutter/cocoon/5036 with Pull request flutter/cocoon/5036 could not be merged: Required status check "ci.yaml validation" is expected..

[Piinks]

Hmmm I can't rebase, it's already up to date. Not sure what is up with ci.yaml here