test: isolate store singleton from cross-file bun leaks

[margaretjgu]

Summary

• Fixes the Test Bun CI failure on main, which started at commit 2297c98 (fix: respect --json flag on --help for root and groups #373) and fails at test/config/store.test.ts:25 (store > getResolvedConfig > returns undefined before any config is set).
• Root cause: src/config/store.ts is a module-level singleton. bun test runs all test files in a shared process (and distributes them across concurrent workers that share module state), unlike node --test, which isolates each file. Sibling suites such as test/lib/es-client.test.ts leave _config set to { context: {} } in their afterEach, so when store.test.ts's first test runs after one of them in the same worker, the initial-state assertion fails. The node runner never sees this because it isolates files.
• Commit 2297c98 added a large batch of tests to test/cli.test.ts and test/factory.test.ts, which perturbed bun's per-worker file scheduling enough to surface this pre-existing latent leak.
• The fix resets the store singleton via _testResetConfig() in a beforeEach (in addition to afterEach), so the test no longer depends on residual global state and is order-independent under both bun and node.

Test plan

• npx tsc --noEmit passes
• npx tsx --test test/config/store.test.ts passes (node runner)
• bun test test/lib/es-client.test.ts test/config/store.test.ts passes (bun, polluter ordering)
• CI Test Bun job is green on all three OS runners

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	10.61s
✅ REPOSITORY	gitleaks	yes		no	no	60.79s
✅ REPOSITORY	git_diff	yes		no	no	0.77s
✅ REPOSITORY	secretlint	yes		no	no	33.16s
✅ REPOSITORY	trivy	yes		no	no	18.57s
✅ TYPESCRIPT	eslint	1		0	0	3.65s

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