ci: add check to prevent invisible Unicode characters (ZWSP/bidi) in source

Summary

PR #32134 removed three U+200B (zero-width space) characters from a comment in internal/plugin/plugin.go. The characters were invisible, had no effect on behavior, but caused Renovate to emit a repo-wide warning for every project that vendors helm.sh/helm/v4:

⚠️ WARN: Hidden Unicode characters have been discovered in file(s) in your repository...

Proposal

Add a CI step that fails if any invisible/dangerous Unicode codepoints are found in the source tree. A one-liner like:

grep -rP '[\x{200B}-\x{200D}\x{FEFF}\x{202A}-\x{202E}\x{2066}-\x{2069}]' --include='*.go' . && echo 'FAIL: invisible Unicode found' && exit 1 || true

This covers:

• U+200B–U+200D: zero-width spaces
• U+FEFF: BOM / zero-width no-break space
• U+202A–U+202E: bidirectional text controls
• U+2066–U+2069: bidirectional isolates (the "trojan source" class)

Alternatively, a golangci-lint plugin or a standalone tool such as bidichk could be wired into the existing lint pipeline.

Motivation

• Prevents the same class of issue from silently reappearing.
• Protects downstream vendor/ consumers from noisy Renovate warnings that can't be suppressed per-path.
• Guards against "trojan source" style attacks (bidi overrides in code comments/strings).