lisa/fix-screenshots-only-as-image-types

[gusfcarvalho]

automated implementation by lisa.

[Copilot]

Pull request overview

Restricts evidence submissions of type "screenshot" to image file types by validating both the declared MIME MediaType and the file extension against an allow-list, returning a new ErrInvalidEvidenceSubmission error mapped to HTTP 400 by the handler.

Changes:

• Add validateEvidenceSubmissionFileType and a small fileExtension helper invoked from validateEvidenceRequirements to enforce image-only types for screenshot evidence.
• Introduce ErrInvalidEvidenceSubmission sentinel error, surfaced as a 400 response in StepExecutionHandler.TransitionStep.
• Add table-driven tests covering allowed media types/extensions, rejected document types, and unaffected non-screenshot evidence.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/workflow/step_transition.go	Adds screenshot file-type validation logic and new sentinel error.
internal/workflow/step_transition_test.go	Tests accept/reject paths for screenshot evidence and confirms broader types remain allowed for non-screenshot evidence.
internal/api/handler/workflows/step_execution.go	Maps the new error to HTTP 400 in the transition endpoint.

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[gusfcarvalho]

lisa: implementation done. no actionable copilot review threads found.