Respect explicit worker execution models

[SAY-5]

--tx=popen//execmodel=thread currently fails while constructing its worker specification because NodeManager prepends a second execmodel value. Both the constructor and setup_node() now supply main_thread_only only when the specification has no explicit model. The controller still uses main_thread_only.

This fixes the duplicate-key bug reported in #1069 (comment). It does not implement automatic gevent monkey-patching from the original issue.

Tests cover string and XSpec inputs, direct gateway setup with a real local worker, and an end-to-end thread-worker run. Before the change, eight cases fail with the duplicate-key error and four controls pass; afterward all twelve pass.

Validation on macOS ARM64, CPython 3.13.9, pytest 8.4.2 and execnet 2.1.2:

• Complete test_workermanage.py and test_remote.py: 49 passed, one SSH-dependent skip, three existing expected failures.
• Complete test_plugin.py: 37 passed, one optional-psutil skip.
• Pinned Ruff 0.16.5 lint and formatting checks passed for all 32 Python files.

Gevent and eventlet cases check specification preservation without executing those optional backends. The full suite, other interpreter/platform combinations and remaining pre-commit hooks were not run locally.

AI assistance: Codex assisted with the implementation, regression tests, review and this description.

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[RonnyPfannschmidt]

the failures are expected and will be fixed elsewhere, thanks