Reduce boilerplate in e2e.yml using matrix strategy

[Copilot]

Two nearly identical jobs (versions-macOS-15, versions-macOS-26) duplicated all steps, differing only in runner and Xcode versions.

Changes

• Merged into single versions job using a 2D matrix (runner × xcode-version)
• Shared versions (latest, latest-stable) expressed as cross-product across both runners
• Runner-specific numeric versions added via include entries

strategy:
  matrix:
    runner: [macos-15, macos-26]
    xcode-version: [latest, latest-stable]
    include:
      # https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md#xcode
      - runner: macos-15
        xcode-version: '16.2'
      - runner: macos-15
        xcode-version: '16.4'
      # https://github.com/actions/runner-images/blob/main/images/macos/macos-26-Readme.md#xcode
      - runner: macos-26
        xcode-version: '26.2'
      - runner: macos-26
        xcode-version: '26.3'
  fail-fast: false

All 8 original test combinations are preserved.

[Goooler]

Addressed to maxim-lobanov#98.