Add comprehensive edge case tests for NoSQL operator detection

[Copilot]

Addresses feedback on PR #35 requesting broader test coverage for NoSQL injection operator detection boundary cases.

Changes

• Added TestNoSQLInjectionDetectorEdgeCases with 26 test cases covering:
  • Dollar signs not at operator boundaries: "price$", "$123", "price$value"
  • Unknown operators with valid syntax: "$unknown", "$hello"
  • Operators at string start and after all delimiter types: {, [, ,, :, ", ', (, whitespace
  • Case normalization: "$NE", "$Ne" correctly detected (operators are lowercased)

Tests validate the detector correctly distinguishes legitimate dollar sign usage from injection patterns based on the isNoSQLOperatorBoundary logic.

// Should not detect - dollar not at boundary
detector.Detect("price$value") // nil

// Should detect - dollar after delimiter
detector.Detect(`{"$ne":null}`) // ErrNoSQLInjectionDetected

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[Copilot]

Pull request overview

This PR adds comprehensive edge case test coverage for NoSQL injection operator detection, addressing feedback from PR #35 that requested broader boundary case testing.

Changes:

• Added TestNoSQLInjectionDetectorEdgeCases with 26 test cases covering dollar sign positions, unknown operators, various delimiters, and case normalization
• Tests validate the detector correctly distinguishes legitimate dollar sign usage from injection patterns based on operator boundary logic

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