fix(middleware): honor all-types compress wildcard

[happysnaker]

Summary

• make NewCompressor(..., "/*") behave as an actual all-types wildcard
• keep the existing wildcard parsing model and fix only the matching side
• add regression coverage for both application/* and text/* responses

Problem

NewCompressor(5, "/*") is currently accepted as a valid wildcard pattern, but it silently does not work.

During setup, the pattern is normalized into an empty wildcard key via strings.CutSuffix("/*", "/*"), which is fine as an internal sentinel. The problem is in isCompressible(): wildcard matching only checks the top-level media type prefix (for example text or application) and never treats the empty key as a match-all sentinel.

That means the following configuration looks valid but compresses nothing:

middleware.NewCompressor(5, "/*")

Changes

• treat an empty wildcard key as "match any top-level media type" during content-type checks
• add a focused regression test showing that "/*" now compresses both application/json and text/html; charset=utf-8

The patch is intentionally narrow: it does not change the wildcard syntax rules, only makes the already-accepted "/*" form behave consistently.

Closes #868

[happysnaker]

Maintainer context:

• this keeps the existing wildcard syntax exactly as-is and only fixes the already-accepted "/*" form on the matching side
• NewCompressor(..., "/*") already normalizes into an empty wildcard key today, so the narrow intent here is just to make that sentinel behave consistently instead of silently matching nothing
• I kept the regression coverage focused on real response types (application/json and text/html; charset=utf-8) to show the intended runtime behavior without broadening the wildcard model further