Expand test coverage for secrets detection and redaction package

[Copilot]

Addresses code review feedback on PR #42 identifying missing test coverage for error conditions, edge cases, configuration options, and nested structure handling.

Tests Added

Error Handling

• ErrSecretInputTooLong for all detection methods
• Invalid configurations for both SecretDetector and Redactor (empty/invalid masks, invalid depths, empty patterns/keys)

Edge Cases

• Empty strings, whitespace-only input, nil/empty collections
• Max recursion depth enforcement preventing over-deep traversal

Configuration Options

• WithSecretPattern / WithSecretPatterns - custom pattern detection
• WithSecretMaxLength / WithSecretMask - input limits and custom masking
• WithRedactionKeys - case-insensitive key matching
• WithRedactionMaxDepth - recursion depth control

Complex Structures

• Maps nested 3+ levels deep with sensitive keys at each level
• Slices containing maps with mixed sensitive/non-sensitive fields

Coverage Impact

Test coverage: 72.6% of statements (up from minimal baseline)

All 16 test functions with 29 subtests passing.

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[Copilot]

Pull request overview

This PR significantly expands test coverage for the secrets detection and redaction package, addressing code review feedback on PR #42. The changes add comprehensive test coverage for error conditions, edge cases, configuration options, and complex nested structure handling, bringing test coverage from minimal baseline to 72.6%.

Changes:

• Added 16 test functions with 29 subtests covering error handling, edge cases, and configuration options
• Validated error conditions for input length limits and invalid configurations
• Tested complex nested structures including 3+ level deep maps and slices containing maps

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