[Studio] Reject blank name and endpoint in instance updates

[yx9o]

Problem

POST /api/instances/update applies non-null name and endpoint values without checking whether they are blank.

An existing instance can therefore be saved with whitespace-only required fields, which is inconsistent with instance creation validation.

Expected behavior

• Reject non-null blank instance names with HTTP 400.
• Reject non-null blank instance endpoints with HTTP 400.
• Leave the existing instance unchanged when validation fails.
• Continue treating null fields as omitted fields during partial updates.
• Preserve normal updates with valid values.

Proposed fix

Validate the supplied name and endpoint before modifying the existing instance, and add regression tests covering blank fields and mutation prevention.