feat(tests): migrate RoleManager tests to Swift Testing (phase 1)

[zamderax]

• Convert RoleTests to Swift Testing (@Suite/@test + #expect)
• Keep all test logic identical; no production code changes
• Bump swift-tools-version to 6.0 to use built-in Testing

Note: This depends on #33 (Swift 6 CI). CI will pass after #33, and we can migrate remaining tests in small follow-ups.

[hsluoyz]

@zamderax plz fix CI error:

[zamderax]

Pushed a small fix to address the CI crash: when the effect stream is created with cap == 0, DefaultEffector previously asserted and crashed under Swift 6 + TSAN. I changed it to return a completed stream with res=false instead of asserting.\n\nThis PR remains tests-only in spirit; the change is a minimal safety improvement to prevent crashes in empty-policy cases. If you prefer this fix in a separate PR, I can split it out and point this PR to depend on it.

[hsluoyz]

@zamderax still not fixed

[zamderax]

Ah I see I'll address the CI failures by reverting swift-tools-version to 5.3 in this tests-only PR. The GitHub Action is already running on Swift 6 (see #33), so the Swift Testing module is available to the toolchain even with a 5.3 manifest, and RoleTests compiles and runs.

This avoids triggering Swift 6 strict concurrency checks across the legacy codebase in the same PR. We can migrate production code for full Swift 6 sendability in separate, focused PRs.

Also included a small safety fix to DefaultEffector for cap == 0 (prevents TSAN crash).

[hsluoyz]

🎉 This PR is included in version 1.6.0 🎉

The release is available on GitHub release

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