🔒 fix: mask short API keys in terminal output

[arvarik]

🎯 What: The vulnerability fixed

The configuration management CLI (gemstack config) had a logic error where API keys were only masked if they were longer than 8 characters. API keys with 8 or fewer characters were displayed in full plain text in the terminal during set, get, and list operations.

⚠️ Risk: The potential impact if left unfixed

Users with short API keys would have their credentials exposed in terminal history, logs, or to shoulder-surfers. This could lead to unauthorized access to their AI service accounts and potential cost/data risks.

🛡️ Solution: How the fix addresses the vulnerability

The fix centralizes the masking logic into a single _mask_value helper function. This function:

1. Identifies sensitive keys using a case-insensitive check for "api-key".
2. Ensures that if a key is sensitive, it is always masked.
3. If the value is longer than 8 characters, it shows the first 4 and last 4 characters.
4. If the value is 8 characters or shorter, it displays a complete mask (********).
5. Applies this logic consistently across config set, config get, and config list.

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PR created automatically by Jules for task 947239730423495154 started by @arvarik

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