fix(frontend): Surface grader validation errors in test workspace

[mattbrailsford]

Summary

• Saving a test without graders previously showed only an "x" on the save button — no on-form indication of the failed rule.
• uai-grader-config-builder now renders an invalid placeholder color and validation message once the user has submitted (driven by pristine === false && validity.valid === false).
• The Grading workspace tab gets a red ! hint badge when the $.graders validation message is present, so users land on the right tab even if they were elsewhere when they hit save.

Changes

• grader-config-builder.element.ts: render invalid styling + message text when invalid and not pristine.
• test-grading-workspace-view.element.ts: drop unused UmbFormControlMixin plumbing; bind builder via umbBindToValidation(this, "$.graders", value).
• test-workspace.context.ts: observe messagesOfPathAndDescendant("$.graders") and mirror messages into view.hints for the Grading tab; cleared automatically when the message list shrinks.
• constants.ts / manifests.ts: lift Grading view alias into UAI_TEST_WORKSPACE_VIEW_GRADING_ALIAS so the context and the manifest share the same source.

Test plan

• Open the test workspace for a new test, click save without adding any graders. Verify the save button shows error state.
• Confirm the Grading tab now shows a red ! badge.
• Switch to the Grading tab — verify the add-grader button is rendered with invalid color and the required message ("At least one grader is required" / localized) is shown beneath it.
• Add a grader, confirm the badge clears and the invalid styling is removed.
• Save successfully and confirm normal workflow still works.

Fixes: #145

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