fix(DTPL-364): Fix pdf_split_hook issues (#340)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **Medium Risk**
> Changes core request/hook dispatch and split-PDF async
execution/cancellation paths, which could affect concurrency behavior
and cleanup semantics under load. Added tests mitigate risk, but
regressions could surface in real-world event-loop/thread-local hook
usage.
> 
> **Overview**
> **Fixes `partition_async()` split-PDF execution to be truly async.**
Split chunk collection now runs via awaited async hook dispatch rather
than spinning up a nested event loop in a worker thread, with response
reassembly offloaded so it doesn’t block the event loop.
> 
> **Adds robust cancellation and resource cleanup.** Introduces async
hook dispatch in `SDKHooks` (sync hooks run in `to_thread` with per-hook
serialization), adds cancellation-aware cleanup in
`BaseSDK.do_request_async`, and hardens `SplitPdfHook` with process-wide
PDF-split setup locking, lazy sync executor creation, chunk-task
cancellation/draining, and cleanup of tempdirs/unconsumed chunk files.
Extensive new unit tests cover ordering, concurrency limits,
cancellation, strict-failure behavior, and non-blocking guarantees.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
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code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: PastelStorm

CHANGELOG.md

@@ -1,3 +1,14 @@
+## 0.43.4
+
+### Enhancements
+
+### Features
+
+### Fixes
+* Route split-PDF `partition_async()` result collection through awaited async hook dispatch instead of creating a nested event loop in a worker thread.
+  Sync-only hooks on the async path now run on a worker thread, so hook code that depends on event-loop-thread `contextvars` or thread-local state should pass that state explicitly.
+* Add cancellation cleanup for in-flight split-PDF chunk tasks and preserve existing sync `partition()` split-PDF behavior with lazy executor creation.
+
 ## 0.43.2
 
 ### Enhancements