Fix get_link_names exception for single-link groups (#160)

[Swastik-Verma]

Fixes #160

Problem

RobotCell.get_link_names() raised an exception when called on a group
consisting of a single link (e.g. an end-effector group with no
configurable joints), instead of returning that link's name.

Root cause

When a group's base link and end-effector link are the same (i.e. the
group has only one link), get_link_names() would still call
iter_link_chain(), which internally tries to compute a shortest path
between two identical nodes. This returns an empty path, which was
incorrectly treated as "no chain found," raising an exception even
though the group is valid.

Fix

Added a check in get_link_names(): if the base link and end-effector
link are the same, the function returns that single link's name
directly, without going through the chain-finding logic.

Testing

Added a test case in test_get_link_names using the endeffector
group on the ur10e_gripper_one_beam fixture, which has a single link
and zero configurable joints — reproducing the original bug scenario.
Verified the fix with:

• invoke test --doctest
• invoke testcodeblocks

Both show the same pre-existing, unrelated baseline failures as before
this change (documented in issue discussion/PR comments if needed),
with no new failures introduced.