Harden Ghidra remote server shutdown

[amattas]

Problem

BinSync's Ghidra integration runs a declib DecompilerServer inside Ghidra and talks to it from a separate UI process. Two lifecycle issues make failures hard to diagnose and can crash or strand Ghidra:

• GUI-backed Ghidra APIs are not safe to call concurrently from arbitrary socket worker threads, but declib did not mark the Ghidra backend as requiring main-thread dispatch. That means remote UI requests could enter Ghidra from worker threads instead of the thread that owns the Ghidra program context.
• DecompilerClient.shutdown_server() sent shutdown_deci, which tears down the decompiler interface but does not ask the server loop itself to stop. The server already has a shutdown_server handler for that, but the client was not using it.

Together, these make the Ghidra remote path fragile: errors can leave the server running, and backend calls can execute on unsafe threads.

Solution

• Mark GUI-backed GhidraDecompilerInterface instances as requiring DecompilerServer main-thread dispatch.
• Keep headless Ghidra direct by clearing that flag for headless=True instances.
• Change DecompilerClient.shutdown_server() to send the server's real shutdown_server request before disconnecting.
• Add focused tests for the shutdown request type and the Ghidra dispatch flag.

Verification

• PYTEST_ADDOPTS='-p no:cacheprovider' conda run -n ghidra python -m pytest tests/test_client_server.py::TestClientServer::test_shutdown_server_requests_server_stop tests/test_ghidra_server_dispatch.py -q -> 2 passed
• PYTHONPYCACHEPREFIX=/tmp/declib-pyc conda run -n ghidra python -m compileall -q declib tests
• git diff --check

Dependency

The BinSync Ghidra lifecycle PR depends on this declib change for the full fix: BinSync can wait on main-thread-dispatch servers and ask the remote server to stop, but declib must expose that Ghidra requires main-thread dispatch and must route shutdown_server() to the server stop handler.

Notes

The broader declib Ghidra integration suite was not used for this PR because this local checkout is missing the external /Users/amattas/GitHub/bs-artifacts/binaries/fauxware fixture. The focused unit tests cover the behavior changed here without requiring that binary.