promhttp/zstd: add unit tests for zstd writer registration

[90ashish]

What does this PR do?

Adds unit tests for the promhttp/zstd package to validate that importing the
package correctly enables zstd support and that the registered writer
produces valid, decompressible output.

Why is this needed?

The promhttp/zstd package relies on an init()-time side effect to register
zstd compression support via internal.NewZstdWriter. Prior to this change,
this behavior was untested.

These tests:

• Assert that zstd support is registered when the package is imported
• Verify that the writer returned by NewZstdWriter produces valid zstd output

Impact

• No production code changes
• Improves test coverage and confidence in zstd support
• No API or behavior changes

Notes for reviewers

The tests intentionally focus on the package’s side-effect contract
(blank import enables zstd support) and avoid any changes to existing logic.