test(test-runner-core): migrate tests to node:test

[bennypowers]

Summary

• Migrate 2 test files (22 tests) from mocha/chai/hanbi to node:test/node:assert/mock
• Replace hanbi.spy() + .handler pattern with mock.fn() (function is callable directly)
• Replace hanbi.stubMethod() with mock.method()
• Replace __dirname with import.meta.dirname
• Replace src/ imports with dist/ imports for native type stripping
• Separate type-only imports with import type
• Increase timeout margins on 2 timing-sensitive scheduler tests for node:test event loop compatibility
• Update test:node script from mocha to node --experimental-strip-types --test --test-force-exit

Test plan

• All 22 tests pass locally on Node 22.22.3
• Tests pass reliably across multiple consecutive runs (no flakiness)
• Lint passes
• Build passes

[changeset-bot]

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Latest commit: c5b9909

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