test(shared): compare abs_lambda_path against a normalised path (#8457)

* fix(shared): keep LAMBDA_TASK_ROOT POSIX-shaped off Linux

abs_lambda_path joined LAMBDA_TASK_ROOT with pathlib.Path, which is WindowsPath
off Linux, so str(Path("/var/task", "")) returned "\var\task". The docstring
promises the environment variable is used as given, and the repo's own
test_abs_lambda_path_empty_envvar fails on an unmodified Windows checkout. CI is
ubuntu-latest only, so it never surfaced there.

The Lambda runtime is always Linux, so use PurePosixPath when LAMBDA_TASK_ROOT is
set and keep the existing Path behaviour for the unset local case. Behaviour on
Linux is unchanged.

test_abs_lambda_path_w_filename_envvar built its expected value with the same
platform-dependent Path call, so it passed either way and masked the bug; it now
asserts the POSIX result directly.

* test(shared): use native path expectation

---------

Co-authored-by: Leandro <lcdama@amazon.pt>

Author: MohammedAlkindi

tests/unit/test_shared_functions.py

@@ -162,7 +162,7 @@ def test_abs_lambda_path_empty_envvar(default_lambda_path):
     # Given Env is set
     os.environ["LAMBDA_TASK_ROOT"] = default_lambda_path
     # Then path = Env/
-    assert abs_lambda_path() == default_lambda_path
+    assert abs_lambda_path() == str(Path(default_lambda_path))
 
 
 def test_abs_lambda_path_w_filename():