test(mesh/session): cover get_session client-fallback + explicit-endpoint paths

[cagataycali]

Problem

strands_robots/mesh/session.py::get_session() had three behavioral branches with no test coverage (lines 628-651, the explicit-endpoint and client-fallback-failure tails). Its sibling _get_zenoh_session_directly already pins the equivalent paths, so the public entry point was the gap. Module coverage sat at 93%.

Change

Adds three behavioral tests to TestSessionLifecycle (zenoh mocked, no broker or install required), asserting outputs/contract rather than internals:

• listener + client both fail: auto-listener path where the listener open and the client-mode fallback both raise a transport error -> get_session() returns None, caches no session, refcount stays 0, and exactly two open() calls are made.
• explicit endpoints succeed (ZENOH_CONNECT set): a single direct open(), session cached, refcount = 1, no listener/client dance.
• explicit endpoints fail (ZENOH_LISTEN set): a transport error on the single explicit open returns None with no silent listener/client retry (loud-on-misconfig contract).

Result

strands_robots/mesh/session.py coverage 93% -> 97% (lines 628-651 now executed). Verified the lines were unexecuted before this change by running the mesh suite against pre-change code. Test-only change; no production behavior altered.

41 passed in 1.27s
strands_robots/mesh/session.py   334   9   97%   87-88, 176, 308, 311, 788-789, 849-850

[yinsong1986]

Summary

Test-only change (+66/-0 in tests/mesh/test_mesh_session.py) that closes the coverage gap on get_session()'s three untested behavioral tails (lines 628-651): listener+client both failing returns None with no cached session and refcount 0; explicit ZENOH_CONNECT opening directly and caching with refcount 1; and explicit ZENOH_LISTEN failing loudly (no silent listener/client downgrade). Tests mock zenoh (no broker or install required), assert on outputs/contract rather than internals (per AGENTS.md "Test behavior, not implementation"), and sit inside TestSessionLifecycle, whose autouse _reset_session fixture restores _SESSION/_SESSION_REFS before and after each test so there is no module-global state leak.

What's good

• Scope-disciplined: no production behavior altered; matches the PR title.
• Env mutation goes through patch.dict("os.environ", ...) rather than raw assignment, so teardown is atomic (AGENTS.md PR #86 testing patterns).
• Each new test pins a distinct contract branch with a clear docstring; the loud-on-misconfig assertion (open.assert_called_once() for explicit endpoints) is the right behavioral pin.
• No host paths, no non-ASCII in the diff.

Verification suggestions

python -m pytest tests/mesh/test_mesh_session.py -k TestSessionLifecycle -q (8 passed locally).