fix(typescript): no loader required under Bun

[mirao]

Fixes #5697

Problem

require: ['tsx/esm'] is mandatory even when running under Bun, which transpiles TypeScript natively. Without it the run aborts with process.exit(1) and the "TypeScript Test Files Detected but No Loader Configured" banner — although the tests run perfectly fine once the check is bypassed.

checkTypeScriptLoader() only string-matches the require array against a hardcoded list of Node loaders; there is no check of whether the runtime already handles TypeScript. So under Bun the require entry exists purely to satisfy that string match, and tsx sits in devDependencies and is imported on every run only to do nothing.

Bun's module.register() is a no-op stub, so tsx's resolve/load hooks are never invoked. Verified with a hooks module that logs on every call: bun --bun produces no hook line at all (and the enum evaluates fine, Bun having transpiled it itself), while node --experimental-strip-types does fire the hooks.

Fix

Return true early from checkTypeScriptLoader() when process.versions.bun is set.

This is deliberately Bun-specific and not "skip the check whenever the runtime can do TypeScript". Node cannot replace tsx here: its native type stripping rejects enums (ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX) and does not resolve extensionless relative imports.

Verification

Ran the minimal reproduction from the issue against this branch — Bun 1.4.2, no require entry, tsx not installed:

Run	Result
Bun, with fix	✔ plain TS scenario with an enum — OK | 1 passed
Bun, fix reverted	loader banner, exit 1
Node, with fix	loader banner (unchanged, as intended)

The Node row is the negative control: the fix is Bun-gated, so Node still demands a real loader — which it must, since native type stripping rejects the enum in that very repro file.

Also confirmed on a real 14-suite Playwright project (~1900 tests): removing the require line and deleting tsx from node_modules entirely, a full browser run passed unchanged — page objects, enums and extensionless relative imports included.

Tests

lib/utils/loaderCheck.js had no unit coverage. Added test/unit/utils/loaderCheck_test.js, which stubs process.versions.bun (mutable under both runtimes) and restores it in afterEach, pinning both the new Bun branch and the pre-existing Node behaviour so the loader list cannot regress silently.

• New file: 5 passing
• Full unit suite (mocha test/unit --recursive): 775 passing, 11 pending, 0 failing
• eslint and prettier clean

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