Potential fix for code scanning alert no. 16: Uncontrolled data used in path expression

[chanhanzhan]

Potential fix for https://github.com/XxxXTeam/Antigravity-/security/code-scanning/16

To fix the problem, we need to robustly validate the user-provided key before it's used to construct a file path. Since the design appears to want the key string as a single filename component, we should ensure that the key does not contain any path separator ("/" or "\") or ".." that could be used for path traversal. This can be done using strings.Contains checks before passing the key into the filename. The best approach is to add an early validation in the Delete function in KeyStore so that any attempt to use a dangerous key is blocked, returning an appropriate error.

Required changes:

• In KeyStore.Delete (lines 101-105), add validation to reject keys containing "/", "", or "..", and return an error if validation fails.
• Optionally, factor the logic into a helper, but since the context is limited, direct checks are best.

No new imports are needed since strings is already imported.

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