Renames the Agents chat stream-silence error from `startup_timeout` to
`stream_silence_timeout` now that the timeout applies to any gap between
provider stream parts, not just first-token startup.
Updates the SDK enum, generated API docs/types, chat error copy, and
Agents UI stories/status labels so the user-facing wording describes a
stalled provider response instead of startup delay.
> **Breaking change:** This is a very minor breaking change for the
Coder Agents API: the public chat error kind enum no longer includes
`startup_timeout`, so clients matching that specific value should handle
`stream_silence_timeout` instead.