Update agent

Update an existing agent. Supports partial updates — only provided fields are changed. Both config and externalConfig are merged onto the stored values, so a partial write preserves keys it omits (notably externalConfig.protocol and externalConfig.cachedAgentCard). To REMOVE a value, send it explicitly as null: a null field clears that field, and externalConfig: null clears the whole object. Nested values replace atomically — sending auth replaces the entire auth block, so switching auth type cannot reuse the previous credential. Agent management responses return a configured auth.credentials value as <redacted>; echoing that placeholder on an update preserves the stored credential only when auth.type is unchanged. Repointing an A2A external agent (endpoint, agentCardUrl, protocol, transport or deviceId) re-fetches and caches the agent card from its new location, as create does, and rejects the update with 400 or 502 when no card can be fetched; an explicitly sent cachedAgentCard is kept as sent, and switching to the runtype-stream protocol discards the card. Moving externalConfig.endpoint to a different host additionally requires re-sending externalConfig.auth: a stored credential is never carried to a host it was not configured against, so a repoint that omits auth is rejected with 400.