fix: stop ignoring committed testdata fixtures

[wcollins]

Description

The blanket testdata/ ignore (mislabeled "Test cache") was swallowing Go test fixtures, which are committed by convention. This forced contributors to git add -f every fixture and recently caused PR #202's tests to panic when fixtures were silently left out of a commit.

Type of Change

• Bug fix

Changes Made

• Remove the blanket testdata/ rule so Go fixtures under testdata/ are tracked normally
• Make the fuzz ignore depth-agnostic (testdata/fuzz/ -> **/testdata/fuzz/) so generated corpora stay ignored at any depth, preserving prior behavior
• Keep *_testdata/ untouched (matches nothing tracked)

Testing

• git check-ignore confirms existing fixtures (e.g. pkg/services/testdata/...) are no longer ignored
• git check-ignore confirms fuzz corpora at root and nested paths remain ignored
• No previously force-added fixtures are affected; future fixtures no longer need -f

Checklist

• Code follows the project's style guidelines
• Self-review of code has been performed
• Commits follow conventional format
• No secrets or credentials committed