Matrix fan out fan in

[rnro]

Add fan-in jobs with stable names to reusable CI workflows

Motivation

• Downstream repos pin required checks to specific job names like
  Linux (6.2) or Xcode 16.3. Every time a Swift/Xcode version is
  added or removed, all downstream repos must update their required
  checks.
• A stable fan-in job that aggregates matrix results means downstream
  repos only need to register it once and never update it again.

Modifications

• scripts/generate_matrix.sh: add a required boolean field to each
  matrix entry — false for nightly-* versions, true for all
  stable versions.
• .github/workflows/swift_test_matrix.yml: add
  continue-on-error: ${{ !matrix.config.required }} to
  execute-matrix so nightly failures are absorbed at the job level
  rather than propagating to the matrix result.
• .github/workflows/macos_tests.yml: add "required": true to all
  stable Xcode entries and "required": false to xcode_latest_beta
  in the inline bash matrix; add continue-on-error to darwin-job.
• Add a required-checks-passed fan-in job to unit_tests.yml,
  cxx_interop.yml, release_builds.yml, and macos_tests.yml with
  stable names Unit tests (All Required), Cxx interop (All Required), Release builds (All Required), and macOS tests (All Required) respectively.

Result

• Downstream repos can register the stable fan-in job name as their
  required check once and never revisit it when Swift/Xcode versions
  rotate.
• Nightly failures do not cause the fan-in job to fail by default; only
  failures in required (stable) matrix entries do.
• Existing per-version job names are unchanged, so existing downstream
  required checks continue to work.
• In the failure case, users will see something like:

One or more required release build matrix jobs failed.
By default, stable versions are required and nightly versions are not.
To diagnose, expand the 'Release builds' job above and look for failing matrix entries.
To change which versions are required, see the linux_*_required / windows_*_required inputs in .github/workflows/release_builds.yml.

See:

• https://github.com/apple/swift-nio/actions/runs/22235262601/job/64325620035?pr=3517 for a success example
• https://github.com/apple/swift-nio/actions/runs/22239259740/job/64339234857?pr=3518 for a failure example