test: Add integration tests for tolerant hydration in mount()

[vertz-advocate]

Summary

mount() with { hydration: "tolerant" } is a critical path for SSR apps. The current implementation in packages/ui/src/mount.ts and packages/ui/src/hydrate/hydration-context.ts handles DOM walking, node claiming, and error recovery — but there are no integration tests verifying the full mount-with-hydration flow.

Missing Tests

Integration tests

• Preserves SSR DOM nodes — mount(App, root, { hydration: "tolerant" }) on a root with existing SSR HTML should NOT clear and re-create the DOM. The original DOM nodes should be the same object references after hydration.

• Attaches event handlers — After tolerant hydration, event handlers (e.g., onclick) should be attached to the existing SSR elements and fire correctly on user interaction.

• Falls back to CSR on mismatch — When the SSR HTML structure does not match what the app function produces (e.g., different tag names, missing elements), mount() should catch the error, log a warning, and fall back to replace mode (clear + re-render). The app should still work after fallback.

Regression tests

• startHydration is actually called in tolerant mode — Verify that startHydration(root) is invoked when mount() is called with hydration: "tolerant" and the root has children. This prevents regressions where the tolerant code path is accidentally bypassed.

Context

• Implementation: packages/ui/src/mount.ts (lines 70-100, tolerant hydration branch)
• Hydration context: packages/ui/src/hydrate/hydration-context.ts
• These tests should live in packages/ui/src/__tests__/ alongside existing mount tests
• Tests need a DOM environment (happy-dom or jsdom)

Acceptance Criteria

• All four test cases above pass
• Tests use real DOM assertions (node identity, event firing), not just "no errors thrown"
• Tests verify the dev-mode warning messages are logged where applicable

[viniciusdacal]

Closing — comprehensive tolerant hydration integration tests exist.

Relevant commits:

• a4344b7 fix: Use tolerant hydration and D1 adapter in PR feat(entity-todo): Phase 3 - SSR + Hydration infrastructure #625

Test file packages/ui/src/__tests__/mount-hydration.test.ts covers: SSR content preservation, browser extension node handling, event handlers on adopted elements, reactive updates after hydration, CSR fallback on empty root, and graceful bail-out on hydration error.