qrexec-agent: avoid blocking on incomplete trigger requests

[iasds]

Summary

• accept local trigger clients without blocking the agent event loop
• track incomplete trigger requests and read their header and body incrementally via ppoll
• bound incomplete requests with a timeout and evict the oldest pending client at capacity
• add socket regression tests for a competing valid request and fragmented request framing

Test plan

• Reproduced the original issue in an AppVM: baseline 0.01s, blocked 3.01s (exit 124), recovered 0.01s.
• gcc -fsyntax-only for agent/qrexec-agent.c using the project include paths and a minimal libvchan API stub; the AppVM lacks the real libvchan development package.
• python3 -m py_compile qrexec/tests/socket/agent.py
• git diff --check

Fixes QubesOS/qubes-issues#11010

[iasds]

Closing this direct release4.3 PR in favor of #239, which targets main. Recent project history shows fixes are merged through main first; maintainers can select an appropriate stable-branch backport after review.