[CodingBreadd]

What

tools/select_tests.py and tools/stubgen.py reconfigure stdout and stderr to UTF-8, but three test call sites read their output with text=True, which decodes with the locale encoding:

• BuildErrorTests.run_tool in tools/tests/test_select_tests.py (3 tests, 10 cases);
• test_dangling_targets_fail_instead_of_selecting_no_tests in the same file;
• test_readonly_second_output_preserves_both_outputs in tools/tests/test_stubgen.py, which runs only on Windows.

On Windows without Python's UTF-8 mode, the locale encoding is the ANSI code page. When the temporary directory holds a character whose UTF-8 bytes the code page cannot decode, such as с (d1 81) in a TEMP like C:\Users\Иван\AppData\Local\Temp under code page 1252, the error message that names the path fails to decode in subprocess's reader thread (UnicodeDecodeError: 'charmap' codec can't decode byte 0x81), stdout/stderr are None, and the assertions raise TypeError.

These call sites now pass encoding="utf-8", as tools/tests/test_audit_dump.py, tools/tests/test_configure_seed.py and other tool tests already do. The tools are unchanged. Same kind of fix as #13269 for test_pr_overlap_export_cache.py; no overlap with it.

Tested

Windows 11, Python 3.14 (ANSI code page 1252), WinLibs GCC 15.2.0 UCRT POSIX SEH toolchain on PATH.

Command	TEMP	main	this PR
python tools/tests/test_stubgen.py	C:\b\l3 тест	28 run, 1 error	28 run, OK (2 skipped)
python tools/tests/test_select_tests.py BuildErrorTests GraphClosureTests	C:\b\l3 тест	7 run, 11 errors (subtests)	7 run, OK
python tools/tests/test_stubgen.py	default	OK	28 run, OK (2 skipped)
python tools/tests/test_select_tests.py	default	OK	42 run, OK

The other test_select_tests.py classes configure a CMake/Ninja build in the temporary directory, which the MinGW toolchain here cannot do in a directory outside the ANSI code page; they are not affected by this change.

• python tools/check_conventions.py --base origin/main: all checks passed.
• Not run here: Linux and macOS, where the locale is normally UTF-8 already.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed under Silent stubs in their library's technical debt file
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source
• Generic behaviour, not specific to one title
• Depends on: none
• Fixes: none
• Unblocks: none
• GPUs tested: n/a
• AI-assisted: yes

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