fix(browser-sdk): clean up Preact roots before test teardown

[roncohen]

Root cause

Both #739 and #742 were ejected from the merge queue after all 205 browser assertions passed: a delayed Preact toolbar render accessed sessionStorage after jsdom teardown. client.stop() does not unmount the UI; removing DOM nodes alone does not cancel Preact work.

Fix

• Unmount feedback and toolbar shadow-root trees before removing their hosts in shared afterEach cleanup.
• Return the toolbar flagsUpdated unsubscribe function from its effect.
• Remove the earlier usage.test.ts toolbar mock: tests now exercise the real toolbar.
• Add two deterministic regressions covering delayed effects and already-queued renders after browser globals disappear. Omitting the unmount makes both fail with the exact ReferenceError.

Validation

• Unmodified main: reproduced the exact sessionStorage rejection in 5 of 40 complete browser-suite runs in a local Linux container (Node 24.13.0, 2 CPUs, CI=true).
• Fixed code: 0 failures in 40 complete browser-suite runs in the same Linux container, with the usage-test toolbar mock removed. Every run exited 0.
• Both GitHub Build & Test checks passed.
• The two deterministic tests pass with unmounting, and both fail with the exact ReferenceError: sessionStorage is not defined when render(null, host.shadowRoot) is omitted (even when the host DOM node is removed).
• Local reproduction logs: /tmp/reflag-flake-linux/repro-baseline/{1..40}.log and /tmp/reflag-flake-linux/repro-fixed/{1..40}.log; fixed exit codes are in repro-fixed/results.txt.
• Reproduction environment: node:24.13.0-bookworm, Docker --cpus=2, immutable root install, then repeatedly run CI=true yarn vitest run --reporter=default from packages/browser-sdk. Native macOS runs did not reproduce the exact error, so the before/after counts above are exclusively Linux.
• Full uncached build and all nine workspace test:ci targets passed locally, including Playwright.
• Formatting and lint passed (existing Next-generated triple-slash warning only).

Follow-up: once merged, requeue #739 and #742.

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