Lazily import pygments in TerminalWriter

[yoshi-taka]

Summary

pygments is no longer imported at module load of _pytest/_io/terminalwriter.py.
It is imported lazily, only when source is actually syntax-highlighted (e.g. when
rendering a traceback with color output). This avoids importing the relatively heavy
pygments package on runs that don't need it — notably runs without markup, such as
most CI executions — slightly reducing pytest's startup time.

Risk analysis / considerations

• The function return annotations (Lexer, TerminalFormatter) are only needed for
  type checkers; the module already uses from __future__ import annotations, so they
  are never evaluated at runtime. They are imported under TYPE_CHECKING.
• Highlighting remains gated by hasmarkup / code_highlight, so behavior is
  unchanged: runs without color output (typical CI) never import pygments, while
  color runs import it once on first traceback, as before.
• No public API is changed. All pygments imports were moved into the methods that
  use them. Because pygments is no longer imported at module level, the previously
  qualified references (pygments.util.ClassNotFound, pygments.util.OptionError,
  pygments.highlight) could only be reached via the pygments module object, which
  would mean importing the whole pygments package inside each of those methods — the
  very thing this change is trying to avoid. So the names are imported directly (e.g.
  from pygments.util import ClassNotFound, from pygments import highlight) and used
  unqualified. This is a naming-only change — the caught exceptions and the highlighted
  output are identical.
• A regression test asserts pygments is not imported when the module is loaded.

Notes on Python versions

• pytest's minimum supported Python is 3.10, so we cannot use the 3.15 lazy import
  keyword (PEP 810, Final, shipped in 3.15): it is a 3.15-only soft keyword and would
  be a SyntaxError on older interpreters.
• PEP 810 also offers a __lazy_modules__ module-level opt-in that is a no-op before
  3.15, but moving the imports into the functions that use them is simpler, works
  uniformly across all supported versions, and matches the approach already used in this
  codebase (e.g. unittest in debugging).
• This change is also robust under the global PYTHON_LAZY_IMPORTS=all mode (already
  exercised by testing/test_assertrewrite.py): a function-local import resolves at
  first use regardless of the global lazy-import setting.

Checklist

• Include new tests or update existing tests when applicable
  (added test_terminalwriter_import_does_not_import_pygments)
• Create a new changelog file (changelog/14948.improvement.rst)
• Add yourself to AUTHORS in alphabetical order — skipped (minor/internal change)
• Documentation for new features — N/A (behavior-preserving refactor, no new feature)