fix(a2a-client): watches no longer hang on a silently dead task stream

Hour-long remote tasks were never delivered: a subscribe watch held one SSE stream for up to a day, the remote
sent nothing while its agent worked without text, and once the quiet connection died on the way (no FIN/RST) the
watch waited on it with no error and no log line.

- A subscription lasts at most 5 minutes (SubscriptionSlice), then GetTask and a new subscription: a pause, not a
  failure. A slice ended mid-read counts as ours even when it surfaces as an I/O error.
- The shared handler connects with TCP keepalive (60 s, every 15 s, 4 probes) and HTTP/2 pings.
- Log lines when a watch starts or is restored, when a slice ends on a running task, when a result is delivered or
  kept; the Telegram host's console log carries a UTC timestamp.
- action=status on a settled task ends its watch, so the result isn't delivered a second time.
- A "context busy" refusal of send says nothing was sent, names the watched task in that context and that its
  result will arrive by itself; refusals never touch a watch.

docs/docker.md

@@ -102,6 +102,7 @@ The compose file maps them into the container.
 
 `TELEGRAM_MODEL` and `LOG_LEVEL` are read by the compose file only, which passes them to the bot as `Telegram__Model`
 and `Logging__LogLevel__Default`; `INSTALL_PLAYWRIGHT` and `CLAUDE_CODE_VERSION` are build arguments (see above).
+Every log line starts with its UTC time (`2026-10-16 13:48:00 UTC info: ...`, `Logging:Console` in `appsettings.json`).
 
 > **Never set `ANTHROPIC_API_KEY`, `ANTHROPIC_AUTH_TOKEN`, `ANTHROPIC_BASE_URL`, `CLAUDE_CODE_OAUTH_TOKEN`,
 > `CLAUDE_CODE_USE_BEDROCK` or `CLAUDE_CODE_USE_VERTEX`.** The Claude Code provider runs on a Claude subscription only