Fix/user validation error handling

[hariomtiwari-work]

What does this PR change?

Improves validation and error handling for pctl create user.

Changes

• Added email format validation before API call
• Improved error handling to surface actual backend error instead of misleading 500

Result

• Invalid input like pctl create user abc now returns:
  "invalid email format"
• Backend errors are displayed clearly

Context

While reproducing issue #310, it was observed that:

• CLI allowed invalid input
• Backend returned 400 but CLI showed 500

This change improves UX and debugging clarity.

Does the PR depend on any other PRs or Issues? If yes, please list them.

• Issue #310

Checklist

I confirm, that I have...

• [ .] Read and followed the contributing guide in CONTRIBUTING.md
• Added tests for this PR
• [ .] Formatted the code using go fmt (if applicable)
• Updated documentation on the Paralus docs site (if applicable)
• Updated CHANGELOG.md