Write redirected and piped output as UTF-8, and stop the suite inheriting colour settings

[tleonhardt]

Two independent fixes, both already reviewed and merged to consolidated_toolbar in #1752.
Ported here because main is the next patch release and both problems are present on it
today. The third commit from #1752 — a CommandToolbar race — is not included, because
that module does not exist on main.

1. Redirection and piping fail on current Windows (user-visible data loss)

Command output is rendered by Rich and contains box-drawing characters, but redirection
targets and pipes are opened with the locale's encoding. On Windows that is the ANSI code
page, and no cp125x code page can represent them, so redirecting raises
UnicodeEncodeError and the user is left with an empty file plus advice to set
PYTHONIOENCODING.

Measured on main at ca0ddbe7, running help > out.txt under a non-UTF-8 default encoding:

file size: 1 bytes
contains help text: False

After this change the same probe writes 153 bytes containing the help text.

Who this affects

This is not limited to old systems. A fully updated Windows 11 with default settings is
affected. The character that breaks is U+2500 (─), and encodability by code page is:

Windows ANSI code page	Encodes U+2500?
cp1252 US / Western European	No
cp1250 Central European	No
cp1251 Cyrillic	No
cp1253–1258 Greek, Turkish, Hebrew, Arabic, Baltic, Vietnamese	No
cp932 / cp936 / cp949 / cp950 CJK	Yes

Only the CJK double-byte code pages escape, because they happen to include box drawing.

By interpreter, sys.flags.utf8_mode is 0 on Python 3.11, 3.12, 3.13 and 3.14, and 1
on 3.15 — PEP 686 makes UTF-8 mode the default there. So every Python version this project
currently supports below 3.15 is affected.

The other escapes are Windows' "Beta: Use Unicode UTF-8 for worldwide language support"
option, which is off by default, and setting PYTHONUTF8=1 or -X utf8.

In short: a user on Windows 11, US locale, Python 3.13, typing help > out.txt in any cmd2
app loses their output. That is the common case, not an edge case.

main's own tests do not catch this — nothing there redirects Rich-heavy output and reads it
back, so the suite passes while the product is broken. This PR adds tests that do.

2. The suite inherits the caller's colour environment

Rich and cmd2 consult several environment variables when deciding whether to emit styling,
and the suite inherits them. Exporting any one makes large numbers of unrelated tests fail
depending on who runs the suite — measured on main:

Variable	Tests failed
NO_COLOR=1	15
FORCE_COLOR=1	53
TTY_COMPATIBLE=1	53

The failures look like product regressions, which makes them expensive to diagnose; this
cost a reviewer and me a full round trip before the cause was found. An autouse fixture now
neutralizes them, and a guard test fails if any reaches a test again. Tests that exercise
these variables still set them explicitly, because a test's own monkeypatching runs after
the fixture.

Verification

Full suite under every environment that previously broke it:

baseline           1863 passed, 2 skipped
NO_COLOR=1         1863 passed, 2 skipped
FORCE_COLOR=1      1863 passed, 2 skipped
TTY_COMPATIBLE=1   1863 passed, 2 skipped
non-UTF8 locale    1863 passed, 2 skipped

make check, make test and make docs-test all pass. Reverting either encoding change
fails the new tests.

The pipe test runs its filter through sys.executable rather than cat, matching the
convention in the existing pipe tests — cmd.exe has no cat, and this fix exists for
Windows. Note that CI would not have caught that: GitHub's Windows runners have Git for
Windows' utilities on PATH.

Note for merging

These commits also exist on consolidated_toolbar via #1752, so the same change appears in
both histories. The content is identical, so the eventual merge resolves cleanly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.63%. Comparing base (ca0ddbe) to head (c7436ed).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1753      +/-   ##
==========================================
- Coverage   99.64%   99.63%   -0.02%     
==========================================
  Files          23       23              
  Lines        5973     5973              
==========================================
- Hits         5952     5951       -1     
- Misses         21       22       +1     

Flag	Coverage Δ
unittests	99.63% <100.00%> (-0.02%)	⬇️

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[tleonhardt]

@kmvanbrunt @bambu This is a bug fix to merge to main for fixing some edge cases on older Windows systems where utf-8 isn't the default encoding. This manually sets utf-8 as the encoding when piping or redirecting to ensure any Rich text styling is supported. Everything LGTM. But if either of you have time for a quick review, great.