fix!: rename chat stream silence timeout error (#25973)

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.

coderd/apidoc/docs.go

@@ -16723,7 +16723,7 @@ const docTemplate = `{
                 "overloaded",
                 "rate_limit",
                 "timeout",
-                "startup_timeout",
+                "stream_silence_timeout",
                 "auth",
                 "config",
                 "usage_limit",
@@ -16735,7 +16735,7 @@ const docTemplate = `{
                 "ChatErrorKindOverloaded",
                 "ChatErrorKindRateLimit",
                 "ChatErrorKindTimeout",
-                "ChatErrorKindStartupTimeout",
+                "ChatErrorKindStreamSilenceTimeout",
                 "ChatErrorKindAuth",
                 "ChatErrorKindConfig",
                 "ChatErrorKindUsageLimit",

coderd/apidoc/swagger.json

@@ -15049,7 +15049,7 @@
 				"overloaded",
 				"rate_limit",
 				"timeout",
-				"startup_timeout",
+				"stream_silence_timeout",
 				"auth",
 				"config",
 				"usage_limit",
@@ -15061,7 +15061,7 @@
 				"ChatErrorKindOverloaded",
 				"ChatErrorKindRateLimit",
 				"ChatErrorKindTimeout",
-				"ChatErrorKindStartupTimeout",
+				"ChatErrorKindStreamSilenceTimeout",
 				"ChatErrorKindAuth",
 				"ChatErrorKindConfig",
 				"ChatErrorKindUsageLimit",