Address review feedback on the pluggable benchmark provider API. The initial design mirrored the coverage provider (a `getProvider` factory plus a `BenchmarkProviderModule` wrapper), which was a special case that benchmarks — running only in tests — don't need. - Drop `BenchmarkProviderModule` and `getProvider`: a custom provider module now default-exports a `BenchmarkProvider` directly. - Reuse the existing `TestModuleRunner` interface instead of the bespoke `BenchmarkProviderLoader`. - Remove the `TestRunner.runBenchmarks` override hook, now redundant with the provider API, and inline `tinybench.run()` in the default provider.