fix(coderd/agentapi): reject missing RPC messages (#29150) - #29358

Reject `UpdateLifecycle`, `UpdateStartup`, and `BatchUpdateMetadata`
requests with a missing sub-message using `InvalidArgument` instead of
panicking. The whole metadata batch is validated before any value is
trimmed or queued.

Adds tests for each handler, a mixed metadata batch, and one DRPC
round-trip confirming the code propagates and the connection stays
usable.

Refs PLAT-596

Reviewed and updated by Coder Agents on behalf of @dylanhuff-at-coder.

(cherry picked from commit 9c58283)