Explain missing fields when connecting a product UI #387

🤖 Generated by the Agentic Engineer

Problem

During a public compatibility-host trial, a manifest containing url and contract but missing title was rejected with the same generic shape message used for unknown properties. The refusal was correct, but the message did not identify the missing field. This makes a first connection harder to repair without reading the protocol document.

Outcome

Give publishers safe, actionable feedback for missing required fields in the public UI manifest and its contract. Clarify that the host accepts the public descriptor's ui object, which includes url, title and contract.

Acceptance criteria

• Missing top-level and contract fields identify the fixed required field names without echoing supplied values or unknown properties.
• Invalid, oversized and unknown-property inputs remain rejected before iframe navigation; existing URL, protocol, origin and grant validation stays intact.
• The compatibility host revokes an open surface when an invalid edit is submitted, explains the correction, and can reopen the corrected manifest.
• Browser tests reproduce the original feedback problem, exercise recovery and prove rejected inputs do not navigate a product frame.
• The form help and protocol documentation describe the accepted public manifest shape.

Refinement

Priority: Medium. Publishers encounter this on their first compatibility-host connection. Clear feedback reduces trial friction without broadening access or depending on independent-provider adoption evidence. No deadline or production outage is asserted.

Effort: Low. One shared validator, form help, documentation and focused browser coverage; delivery includes the full repository validation and exact-head review/CI. No schema or deployment change is required.

Prerequisites: the public host is available, the UI protocol library already validates these fields, and no open source PR currently addresses this failure. This is an independently useful bug fix; it does not decide the outstanding adoption or release-flag outcomes.