refactor(process): extend preexec_fn→start_new_session swap to remaining Popen sites

PR #8399 replaced preexec_fn=os.setsid with start_new_session=<bool> in
tools/environments/local.py to use CPython's thread-safe POSIX fastpath
instead of a between-fork-and-exec callback. Apply the same swap to the
other three files covered by tests/tools/test_windows_compat.py so all
Popen call sites use a consistent idiom.

Also:
- Strip a trailing-whitespace line introduced by the original PR.
- Update CONTRIBUTING.md + website/docs/developer-guide/contributing.md
  to recommend start_new_session in the cross-platform process management
  section.
- Fix a stale docstring reference in tools/environments/base.py (_popen_bash).
- Extend test_windows_compat.py with a parallel regression guard that
  rejects bare start_new_session=True (must be gated on _IS_WINDOWS).

Scope note: this is code hygiene, not a fix for #8340 — the two forms
invoke the same setsid() syscall, so this swap alone does not change
behavior for the 'setsid ... & disown' hang scenario in that issue.
#8340 remains open.

website/docs/developer-guide/contributing.md

@@ -129,8 +129,7 @@ except UnicodeDecodeError:
 
 ```python
 import platform
-if platform.system() != "Windows":
-    kwargs["preexec_fn"] = os.setsid
+kwargs["start_new_session"] = platform.system() != "Windows"
 ```
 
 ### 4. Path separators