feat(ui): add validation for form fields

[PetrBulanek]

Summary

This PR fixes inconsistent required-field validation across UI form components.

• Aligns UI field implementations with SDK required definitions.
• Ensures all required fields enforce validation.
• Adds consistent invalid states and inline error messages for missing required input.
• Prevents submission when required fields are empty.

Users now receive clear, consistent visual feedback for required fields across all form types, eliminating silent failures and invalid submissions.

Linked Issues

Closes: #1900

Documentation

• No Docs Needed:

If this PR adds new feature or changes existing. Make sure documentation is adjusted accordingly. If the docs is not needed, please explain why.

[gemini-code-assist]

Summary of Changes

Hello @PetrBulanek, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the robustness and user experience of UI form components by implementing consistent and comprehensive required-field validation. It standardizes how validation rules are applied, ensures clear visual feedback for users, and prevents invalid data submissions, leading to a more reliable and intuitive form interaction across the application.

Highlights

• Consistent Validation: Inconsistent required-field validation across UI form components has been fixed, aligning UI field implementations with SDK required definitions.
• Enhanced User Feedback: All required fields now enforce validation, displaying consistent invalid states and inline error messages for missing input.
• Submission Prevention: Form submission is now prevented when required fields are empty, improving data integrity and user experience.
• New Validation Hook: A new useFormFieldValidation hook has been introduced to centralize and standardize validation logic across various form fields.
• Dependency Update: The cachetools dependency has been added to uv.lock files for both agentstack-cli and agentstack-server.

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[gemini-code-assist]

Code Review

This pull request does a great job of implementing consistent required-field validation across the UI form components. The introduction of the useFormFieldValidation hook is an excellent abstraction that centralizes validation logic and improves maintainability. The changes ensure that forms provide clear, immediate feedback to the user, preventing invalid submissions. The code is well-structured and the refactoring of SDK schemas is clean. I've included a couple of suggestions to further improve code quality and component design.

[kapetr]

Looks great. One thing though - I'd consider adding an indicator for required fields and/or not disabling the submit button. It seems to me that in the current state, the user has no infmation about why he can't submit the form.

[kapetr]

Just curious, why is this necessary?

[PetrBulanek]

The useEffect calls onChange with the uploaded files to make the form stay in sync with them. And it ran on the initial render as well, which triggered the validation and "This field is required." showed immediately. This seemed like a good solution to me. But maybe you can think of something better, so be sure to let me know. :-)