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 external agent (endpoint, agentCardUrl or protocol) discards the cached agent card, which is re-fetched by POST /agents/{id}/refresh-agent-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.