UpdateContentStandardsResponse is not a discriminated union; inconsistent with all other write operations

[bokelley]

Problem

UpdateContentStandardsResponse has optional standards_id?: string and errors?: Error[] on a single flat type. An agent cannot determine success vs. failure by structural inspection — it must check whether errors is present and non-empty.

Every other write operation in the protocol uses strict discriminated unions:

• CreateContentStandardsResponse: CreateContentStandardsSuccess | CreateContentStandardsError
• CalibrateContentResponse: CalibrateContentSuccess | CalibrateContentError
• CreateMediaBuyResponse: CreateMediaBuySuccess | CreateMediaBuyError
• etc.

This inconsistency breaks agents that handle all write responses with the same structural pattern.

Proposed fix

Replace UpdateContentStandardsResponse with a discriminated union matching the established protocol pattern:

type UpdateContentStandardsResponse = UpdateContentStandardsSuccess | UpdateContentStandardsError;

interface UpdateContentStandardsSuccess {
  success: true;
  standards_id: string;
  // updated standards object
}

interface UpdateContentStandardsError {
  success: false;
  errors: Error[];
}

[bokelley]

Resolved in #1252.