Fix CLI binary workflow runner labels

[leehack]

Summary

• move the macOS x64 CLI binary job from retired macos-13 to macos-15-intel
• pin Windows binary job to windows-2025 instead of windows-latest
• update upload-artifact to v6 for the current Node runtime

Validation

• git diff --check
• YAML parsed with Ruby Psych

This is needed to unblock the pending CLI binary assets for mcp_dart_cli-v0.2.0-dev.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.75%. Comparing base (6cd000f) to head (a07d0a3).
⚠️ Report is 1 commits behind head on dev/2026-07-28-rc.

Additional details and impacted files

@@                Coverage Diff                 @@
##           dev/2026-07-28-rc     #276   +/-   ##
==================================================
  Coverage              84.75%   84.75%           
==================================================
  Files                     70       70           
  Lines                  15364    15364           
==================================================
  Hits                   13022    13022           
  Misses                  2342     2342           

Flag	Coverage Δ
cli	76.73% <ø> (ø)
unittests	88.24% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.