Add Null Checks and Enhanced Error Handling to admin.js Form Handler

[rakdutta]

Summary of Changes:

• Updated mcpgateway/frontend/admin.js to add null/undefined checks in all form handlers, in line with issue #668.
• Introduced optional chaining and default error messages to handle cases where result.message might be missing.
• Ensured all error messages fall back to a sensible default if the backend response does not include a message.
• Improved robustness and user experience by preventing runtime errors due to unexpected null or undefined values.

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Testing Instructions

These steps validate changes for handleGatewayFormSubmit and handleEditGatewayFormSubmit.
Repeat similar tests for other form handlers.

• Locate the line in admin.js:

  const result = await response.json();

• Case 1: Replace with:

  const result = null;

  Expected Result:

  • "Failed to add gateway"
  • "Failed to edit gateway"

• Case 2: Replace with:

  const result = { success: false };

  Expected Result:

  • "Failed to add gateway"
  • "Failed to edit gateway"

[crivetimihai]

✅ Successfully rebased PR #726 onto latest main

Changes Review: The PR improves error handling in mcpgateway/static/admin.js by:

• Adding null/undefined checks using optional chaining (result?.message)
• Providing specific fallback error messages for each form handler
• Ensuring consistent error handling across all 9 form submission functions

Code Quality:

• ✅ Clean implementation following modern JS best practices
• ✅ All web linters pass (HTML, CSS, JS security checks)
• ✅ All Python tests pass (1643 passed, 11 skipped)
• ✅ Successfully rebased with minimal conflicts (only whitespace issues)

The PR correctly addresses issue #668 and improves the robustness of the admin UI error handling.