AI Preferences: Add show/hide visibility toggle for API key field

[Sudeep-A-Kulkarni]

Summary

The API key field in Preferences → AI has no visibility
toggle. Users cannot verify what they typed without the
key being permanently hidden.

Suggested by

This issue was suggested by maintainer @ThiloteE on the
forum:
https://discourse.jabref.org/t/ai-preferences-add-test-connection-button-and-api-key-show-hide-toggle/9057/2

Current Behavior

API key field is always masked with no way to reveal it.

Expected Behavior

A show/hide toggle button (👁) next to the API key field
allowing users to temporarily verify what they typed —
following industry standard credential handling used by
GitHub, OpenAI console, and AWS.

Proposed Implementation

Replace or wrap the current CustomPasswordField with a
toggle that switches between masked and visible states.

Relevant file:
jabgui/src/main/java/org/jabref/gui/preferences/ai/AiTab.java
Line 53: @FXML private CustomPasswordField apiKeyTextField;

System Information

JabRef v100.0.0, Linux

[ganesh-vk]

Going to work on this along with #15398 together

/assign-me

[jabref-machine]

Thank you for your continued work in JabRef. At this point, you probably already know the basics and the important links. If not, our contributing guidelines is the place you should go to.

At this point in time, you raised 4 pull request(s) of which 75% (3/4) have been merged.

[github-actions]

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