Honor NO_COLOR without suppressing text attributes

[MsfPablo]

Summary

The current StyledObject formatting path in console drops the
attribute escapes (bold, italic, underline, blink, reverse, hidden,
strikethrough) whenever colors are disabled — and NO_COLOR (per
no-color.org) is what disables colors. That contradicts the spec, which
is explicit that only color is suppressed:

Command-line software which adds ANSI color to its output by default
should check for a NO_COLOR environment variable that, when present
(regardless of its value), prevents the addition of ANSI color.

console also currently disables color when NO_COLOR="" is set,
because env::var("NO_COLOR").is_ok() returns true for any value
(including the empty string). Per the spec the presence of the variable
alone should disable color — but a deliberately-empty variable should be
a no-op.

Changes

• Move the attribute escape emission in impl_fmt! (src/utils.rs)
  outside the colors_enabled() / colors_enabled_stderr() gate, so
  attributes are still emitted (with a trailing reset) when colors are
  suppressed.
• Replace env::var("NO_COLOR").is_ok() with a non-empty check
  (env::var_os("NO_COLOR").is_some_and(|v| !v.is_empty())) in
  src/unix_term.rs and src/windows_term/mod.rs, matching the
  spec.
• Add a regression test (test_attrs_survive_colors_disabled) covering
  both behaviors.

Verification

$ make test          # 12 test runs, all pass
$ make lint          # clippy with --deny warnings, clean
$ make format-check  # rustfmt --check, clean

The new test verifies:

• style("foo").bold() still emits \x1b[1m…\x1b[0m when
  colors_enabled() is false.
• style("bar").italic() still emits \x1b[3m…\x1b[0m in the
  same condition.
• style("baz").red() does not emit \x1b[31m and does
  not emit a trailing reset when colors are disabled.

AI disclosure

This PR was drafted with AI assistance (Claude) and self-reviewed. I
read the relevant code paths, reproduced the failing behavior locally
against the crate's existing test harness, and verified the fix does not
change the rendered output for the unaffected code paths.

Fixes #291