fix(ByoImageValidation): ensure BYO agent validation checks both modelName and byoImage fields

[entee28]

This PR closes #2450

[Copilot]

Pull request overview

Fixes a UI validation bug in the Create/Edit Agent page where the BYO “Container image” field could incorrectly show “Container image is required” on blur even when a non-empty value is present (notably in edit mode with a pre-filled image). This aligns the per-field validation payload with how validateAgentFormData distinguishes between Declarative vs BYO agent types.

Changes:

• Update validateField("model", ...) to populate both formData.modelName and formData.byoImage, since the same "model" error slot is reused across Declarative (model) and BYO (container image) flows.
• Add a jsdom regression test that covers the edit-mode BYO blur behavior to ensure the “required” error does not appear when the image is already present.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ui/src/app/agents/new/page.tsx	Ensures per-field validation sets both modelName and byoImage for the shared "model" validation path, preventing false BYO required errors.
ui/src/lib/tests/ByoImageValidation.test.tsx	Adds a regression test validating that a pre-filled BYO image does not trigger the “required” error on first blur in edit mode.

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