feat(vitest-pool-workers): add Istanbul coverage support

[TyposBro]

Summary

Adds Istanbul code coverage support for @cloudflare/vitest-pool-workers. Previously, running tests with coverage.provider: 'istanbul' always reported 0% because coverage data populated inside the workerd runtime never bridged back to the Node.js process.

Changes across 4 files (+163 lines):

• src/pool/module-fallback.ts — Istanbul instrumentation for user source files loaded through the module fallback service (files not handled by Vite transforms). Uses dynamic import of istanbul-lib-instrument (transitive dep of @vitest/coverage-istanbul).

• src/worker/lib/cloudflare/test-runner.ts — Collects globalThis.__VITEST_COVERAGE__ after all test files complete and sends it to the pool via the loopback service.

• src/pool/loopback.ts — Handles /coverage POST route. Merges incoming coverage data (statement, function, and branch counters) into globalThis.__VITEST_COVERAGE__ on the Node.js side where Vitest's coverage provider reads it.

• src/pool/index.ts — Enables coverage instrumentation in executeMethod() when ctx.config.coverage.enabled is true.

How it works

1. When coverage is enabled, the pool initializes an Istanbul instrumenter for the module fallback service
2. Files loaded through module fallback (bypassing Vite transforms) are instrumented with Istanbul
3. Files loaded through Vite transforms are instrumented by Vitest's own Istanbul plugin
4. After tests complete in workerd, the test runner reads globalThis.__VITEST_COVERAGE__ and POSTs it to the pool's loopback service
5. The loopback handler merges coverage data into the Node.js process's global scope
6. Vitest's Istanbul coverage provider picks up the data and generates reports

Testing

Verified with a real project:

• 53 test files, 1,342 tests — all passing
• Coverage reports show real numbers (e.g., 90% for individual files, 17.5% overall)
• Previously reported 0% across the board

Test plan

• Verify single test file produces non-zero coverage
• Verify full test suite (53 files) produces coverage without errors
• Verify coverage data includes correct statement/branch/function counts
• Verify no impact on test runs without coverage enabled

Closes #12589

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🦋 Changeset detected

Latest commit: 9b91902

The changes in this PR will be included in the next version bump.

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[TyposBro]

Addressed latest review feedback in 9b91902:

1. Provider check — enableCoverageInstrumentation() now only activates when ctx.config.coverage.provider === 'istanbul', preventing V8 coverage from being corrupted by Istanbul transforms.

2. CJS named-export detection (from previous review) — Original uninstrumented source is preserved and passed to getCjsNamedExports() so the CJS lexer sees clean code.

Both fixes plus the earlier null-check and retry-prevention fixes are included in this commit.