add form validation for required fields

[iamrajiv]

currently users can click save and test without providing required api keys leading to confusing health check failures we should validate required fields before attempting to save and provide clear error messages

ac:

• validation runs before save check required fields before api calls
• provider specific validation different requirements per provider
  • openai api key required
  • anthropic api key required
  • custom api api key and url required
  • azure api key and url required
• clear error messages specific actionable error text
• multiple errors shown display all validation issues at once
• visual indicators red borders or icons on invalid fields
• save button disabled when validation fails