Add edge case tests for mask function boundary conditions

[Copilot]

Addresses feedback on PR #139 to add missing edge case tests for the mask function, which masks API tokens by showing the first 3 characters and replacing the rest with asterisks.

Changes

• Added test for single-character token (length 1): Verifies token remains fully visible
• Added test for three-character token (length 3): Tests boundary where visible length equals token length
• Added test for four-character token (length 4): Tests first case where masking occurs with one asterisk

These tests cover critical boundary conditions in the masking logic:

func mask(token string) string {
    n := len(token)
    if n == 0 {
        return ""
    }
    visible := min(n, 3)
    return token[:visible] + strings.Repeat("*", n-visible)
}

The new tests ensure correct behavior at the transition points where the function switches from no masking (≤3 chars) to progressive masking (>3 chars).

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[Copilot]

Pull request overview

This PR adds comprehensive edge case tests for the mask function to ensure correct behavior at critical boundary conditions. The mask function is used to safely display API tokens in logs by showing only the first 3 characters and replacing the rest with asterisks.

• Adds three new test cases covering lengths 1, 3, and 4 characters
• Tests the transition point where masking begins (tokens with more than 3 characters)
• Complements existing tests for empty strings, 2-character tokens, and long tokens

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