Closes#31066. Closes#31110.
An unhandled `telegram.error.TimedOut` (or peer `NetworkError` /
`httpx` connection error) propagating to the asyncio event loop killed
the entire gateway process, taking down every profile attached to the
same runner. systemd restarted the service after ~5s but the active
conversation turn was lost.
Public adapter methods (`adapter.send`, `adapter.edit_message`,
`adapter.send_voice`, …) are individually try/except-wrapped on
current main, but at least one async path was reaching the loop with
TimedOut unhandled — the report's traceback ends at the deepest httpx
frame and doesn't pinpoint the caller.
Rather than audit 30+ call sites blind, install a loop-level safety net:
`_gateway_loop_exception_handler` is set as the loop's exception handler
in `start_gateway()` after `asyncio.get_running_loop()`. It classifies
the exception via `_is_transient_network_error()` (walks the
__cause__/__context__ chain, matches on class name so the test suite
doesn't need the real telegram/httpx packages installed). Transient
errors are logged at WARNING with full traceback so the originating
call site stays diagnosable; everything else forwards to
`loop.default_exception_handler` so real bugs still surface.
Tests cover the classifier (known transients accepted, real bugs
rejected, cause/context chain unwrap, cyclic-cause termination) and the
handler (swallow + log warning, forward unknowns, missing-exception
context). One end-to-end test schedules an orphan task raising TimedOut
and asserts `asyncio.run` returns cleanly.
