### Motivation
- Restore the transport cleanup path on SRT session errors so
`SrtTransport::onShutdown` runs its unregister logic and prevents
unbounded growth of weak_ptr entries in the transport manager that could
lead to a memory-exhaustion DoS.
### Description
- Re-enabled shutdown cleanup in `srt/SrtSession.cpp` by capturing `err`
in the delayed callback and calling `transport->onShutdown(err)` from
the `getPoller()->async` lambda.
### Testing
- Ran `cmake -S . -B build` to sanity-check configuration; configuration
failed in this environment due to a missing `3rdpart/ZLToolKit`
submodule and unrelated CMake errors, not due to the patch.
------
[Codex
Task](https://chatgpt.com/codex/cloud/tasks/task_e_69b32b68f8ac8320b6a9b0fad53594c6)
---------
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
