[Studio] fix: validate instance updates

[yx9o]

What is the purpose of the change

Fixes #613 .

Prevent /api/instances/update from accepting blank name or endpoint values and persisting invalid instance data.

Brief changelog

• Reject non-null blank instance names and endpoints with HTTP 400.
• Validate all supplied required fields before modifying the existing instance.
• Preserve partial update semantics for omitted fields.
• Add regression tests for validation failures and mutation prevention.