Fix the macOS test hang: send every byte of virtual masks and XY arrays

[CSSFrancis]

Fixes the macOS CI hang: macos-latest-py3.12 sometimes stalls in test_client.py, then times out every remaining test until the job reaches its 20-minute limit.

Cause

• Client.__sendToSocket could drop data. It sent virtual masks and XY scan arrays in 4096-byte chunks with sock.send(chunk) and ignored the return value. Once a socket has a timeout, send can accept only part of a chunk. macOS's loopback does this regularly. The unsent bytes were dropped, and after a send timeout the loop also skipped the rest of that chunk. The server then waited for mask bytes that never arrived.
• The fake server kept broken connections open. After an error it went back to accept() without closing the connection. The session-wide test client sent every later command into that dead connection, and each test waited out --timeout=60.

In CI this showed up as a hang at test_set_virtual_mask, which sends a 1 MB mask.

Changes

• Client: __sendToSocket uses sendall, with the same timeout as before. The command packets in set_virtual_mask and set_xy_array also use sendall, like the other commands.
• Simulated server: closes a connection after an error, so the client gets an error instead of waiting.
• Test suite: close_port now stops only deapi's own simulated server. Before, running the tests on a machine with DE-Server listening on 13240 terminated DE-Server. Now the run stops with a message instead.

Tests

• New test_fake_server/test_partial_sends.py: wraps the client's socket so send accepts at most 1000 bytes per call, which reproduces the hang on any OS. It covers a virtual mask round trip (followed by one more command, to check the connection is still usable) and the shared send helper over a socket pair.
• Old client: both new tests hang, even on Windows.
• This branch: they pass in about 2 s. The full suite, run locally on Windows with --timeout=60, gives 51 passed and 128 skipped.

Also fixed

test_start_acquisition_scan_disabled also failed intermittently on macOS, and the hang used to hide it. With scan off the fake acquisition is one frame, which takes 1 ms at the suite's 1000 fps. It could finish before the test read acquiring. The test now runs at 2 fps, so the acquisition lasts 0.5 s.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 86.30137% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
deapi/tests/conftest.py	0.00%	8 Missing ⚠️
deapi/client.py	66.66%	1 Missing ⚠️
deapi/simulated_server/initialize_server.py	0.00%	1 Missing ⚠️

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