fix(deps): complete Vitest 4 security migration

[roncohen]

Summary

• Supersedes chore(deps-dev): bump vitest from 1.6.1 to 4.1.11 #723: upgrade workspace Vitest versions to 4.1.11 and align all existing coverage-v8 dependencies.
• Migrate EventSource and both OpenFeature provider constructor mocks to class implementations.
• Isolate toolbar rendering from SDK usage tests to prevent rendering after browser-environment teardown.
• The standalone Cloudflare example is intentionally excluded: its worker pool integration needs a separate compatible migration.

Validation

• Full build, immutable install, formatting, lint: passed.
• All nine workspace unit/type-test suites passed across runs.
• Browser and Node coverage runs passed.
• Playwright passed all 48 tests on one run; other runs hit an intermittent Chromium feedback-widget positioning assertion also observed without the Vitest upgrade.
• GitHub CI required before merging.

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[roncohen]

Resolved conflicts with the merged webpack update in 4c5852f. Preserved Vitest/coverage 4.1.11, Next 15.5.24/16.3.3, and deduplicated webpack 5.111.0 (no reintroduced 5.94.0). Immutable install, full build, formatting and lint passed locally; both GitHub Build & Test checks are green. Re-entered the merge queue. Local Playwright still intermittently hits the previously observed positioning assertion; GitHub browser tests passed.