Fix storeTag regex performance bottleneck

[cdaguerre]

Fixes #696

Warning: This was entirely done by Claude Code

Problem

The storeTag function was using regex matching to check for duplicate cache keys in the stored comma-separated list. This caused ~57% CPU consumption because:

1. A regex was compiled on every Store() call
2. The regex was executed against potentially very long strings containing thousands of cache keys
3. Regex backtracking is O(n×m) complexity

Solution

Replace regex matching with simple string operations (strings.HasPrefix, strings.HasSuffix, strings.Contains) that provide O(n) complexity and leverage Go's optimized strings package.

Changes

• Added containsCacheKey() helper function using fast string operations
• Removed regex parameter from storeTag()
• Removed per-request regexp.MustCompile() calls from Store()
• Updated SurrogateInterface to reflect new signature
• Added unit tests for containsCacheKey() covering edge cases
• Applied same fix to traefik and tyk plugin overrides

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