ci: reduce test matrix and eliminate redundant runs

[HamdaanAliQuatil]

part of #255

• remove macos-14 job (identical to macos-15, doubles CI time)
• merge linux-coverage and windows-coverage into main jobs (re-ran all tests)
• fix redundant test runs: VM and Chrome were tested 2-3x per job
• add concurrency group to cancel superseded runs on same PR branch
• make iOS/Android continue-on-error so emulator flakiness doesn't block

reduces from 10 jobs to 7. fewer test executions.

[jonasfj]

make iOS/Android continue-on-error so emulator flakiness doesn't block

I don't think this means ignore errors. it just means continue running steps, right?

[HamdaanAliQuatil]

make iOS/Android continue-on-error so emulator flakiness doesn't block

I don't think this means ignore errors. it just means continue running steps, right?

I originally meant "keep the rest of the matrix running even if an emulator job flakes", but continue-on-error would make an exhausted iOS/Android failure non-blocking, which is too weak for CI.

After rebasing on top of #257, I'm not keeping this part. iOS/Android now use retry/timeouts instead, and they still fail the workflow if all attempts fail.