[REFACTOR] Remove custom contains in favor of std lib impl

[bashhack]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update
• Go Version Update
• Dependency Update

Description

Replace custom contains helper function with standard library's slices.Contains

Changes

• Removed custom contains() function from helpers.go
• Updated csrf.go to use slices.Contains() instead
• All tests pass successfully

Rationale

This simplifies the codebase by leveraging Go's standard library instead of maintaining a custom implementation. Both have O(n) complexity, so there's no performance impact.

Related Tickets & Documents

N/A

Added/updated tests?

• Yes
• No, and this is why:
  • Changes made simplify long-term maintenance, without altering core application logic
• I need help with writing tests

Run verifications and test

• make verify is passing

⚠️ NOTE: Per contributing guidelines, make verify should be run before PR submission, in running this I noticed failures unrelated to my changes - so while I'm here and since I'm dealing with strings lib changes, I've gone ahead and addressed - though out of scope from my intended change, I hope you don't mind

• make test is passing

[bashhack]

Closing - no response