fix: fall back from incompatible stdin

[MilesCranmerBot]

Summary

• fall back from default input_stream="stdin" to Julia devnull when Python stdin is not an interactive TTY with a valid fd
• preserve explicit/custom Julia stream expressions
• document the fallback and add targeted unit coverage

This should avoid SymbolicRegression.jl trying to start_reading/poll frontend-owned pseudofiles such as marimo's stdin wrapper.

Fixes #832

Tests

• python3 -m py_compile pysr/sr.py pysr/test/test_main.py
• uv run --no-project --python 3.12 --with pytest --with numpy --with pandas --with scikit-learn --with sympy --with juliacall==0.9.28 pytest pysr/test/test_main.py::TestInputStream -q
  • targeted assertions pass (3 passed); on this local machine JuliaCall/PythonCall teardown intermittently exits 139 after pytest reports success

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pysr/sr.py	62.50%	6 Missing ⚠️

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