feat: read output width from the environment when there is no tty

[zen-xu]

Problem

The reporter console is built with force_terminal=True so colors survive being
piped. That flag also makes rich skip its own COLUMNS handling, and with no tty
to measure the width was pinned:

width=None if sys.stdout.isatty() else 200          # console
generate_header_group(session), title=title_msg, width=120   # session header

So in a CI log there was no way to adjust the width — setting COLUMNS had no
effect, because the explicit width= argument overrides it.

Change

Add console_width(), which resolves in the order: no width on a tty (rich
measures it) → COLUMNS → PYTEST_MODERN_WIDTH → 200 as the fallback.
Non-numeric and non-positive values are ignored rather than raising.

PYTEST_MODERN_WIDTH is there so the width can be set without COLUMNS
affecting every other tool in the same shell.

The session header now uses min(120, self.console.width) so it tracks a
narrower console instead of overflowing it.

Verification

console_width() across inputs:

COLUMNS	result
unset	200
100	100
160	160
240	240
abc	200
0	200
-5	200

Test suite compared against main with reruns disabled to remove the
nondeterminism — identical on both: 12 passed, 4 failed, 2 timeout. (Those
failures are the intentional demo fixtures in tests/.) ruff check and
ruff format pass.

Note

The hunk at terminal.py:277 reformats a pre-existing long lambda line and is
unrelated to this change — ruff format insists on it, so reverting it would
fail pre-commit.ci.