GR tests: match router image to server version, surface docker problem

[kaushikpuneet07]

• Default ROUTER_IMAGE is now derived from SERVER_IMAGE (/percona-server:X.Y.Z -> /percona-mysql-router:X.Y.Z), falling back to percona/percona-mysql-router:8.4. A fixed 8.4 router cannot bootstrap against a 9.7 cluster and crash-looped, failing the [router] tests.
• mysqlsh_available() only returns False for a missing binary (exit 127); other docker failures (e.g. socket permission denied) now raise with the real error instead of skipping as "mysqlsh not available".
• README: document the derived router default.

[kaushikpuneet07]

puneet-work@puneet-work:~/server-qa/test_scripts/ps/group-replication$ SERVER_IMAGE=perconalab/percona-server:9.7.2 pytest -v 2>&1 | tee run-972.log
============================= test session starts ==============================
platform linux -- Python 3.10.12, pytest-9.1.1, pluggy-1.6.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /home/puneet-work/server-qa/test_scripts/ps/group-replication
configfile: pytest.ini
testpaths: .
plugins: testinfra-10.2.2, testinfra-6.0.0
collecting ... collected 25 items

test_backup_restore.py::test_full_and_incremental_backup_restore[haproxy] PASSED [ 4%]
test_basic.py::test_replicates_table_across_nodes[router] PASSED [ 8%]
test_basic.py::test_replicates_table_across_nodes[haproxy] PASSED [ 12%]
test_equal_partition.py::test_equal_partition[router] PASSED [ 16%]
test_equal_partition.py::test_equal_partition[haproxy] PASSED [ 20%]
test_majority_loss.py::test_majority_loss[router] PASSED [ 24%]
test_majority_loss.py::test_majority_loss[haproxy] PASSED [ 28%]
test_majority_loss_kill.py::test_majority_loss_by_kill[secondaries-router] PASSED [ 32%]
test_majority_loss_kill.py::test_majority_loss_by_kill[primary_and_secondary-router] PASSED [ 36%]
test_majority_loss_kill.py::test_majority_loss_by_kill[secondaries-haproxy] PASSED [ 40%]
test_majority_loss_kill.py::test_majority_loss_by_kill[primary_and_secondary-haproxy] PASSED [ 44%]
test_primary_isolation_failover.py::test_primary_isolation_failover[router] PASSED [ 48%]
test_primary_isolation_failover.py::test_primary_isolation_failover[haproxy] PASSED [ 52%]
test_primary_shutdown_failover.py::test_primary_shutdown_failover_and_recovery[stop-router] PASSED [ 56%]
test_primary_shutdown_failover.py::test_primary_shutdown_failover_and_recovery[kill-router] PASSED [ 60%]
test_primary_shutdown_failover.py::test_primary_shutdown_failover_and_recovery[stop-haproxy] PASSED [ 64%]
test_primary_shutdown_failover.py::test_primary_shutdown_failover_and_recovery[kill-haproxy] PASSED [ 68%]
test_rejoin_after_scale_up.py::test_rejoin_after_scale_up[router] PASSED [ 72%]
test_rejoin_after_scale_up.py::test_rejoin_after_scale_up[haproxy] PASSED [ 76%]
test_scaling.py::test_scale_up_and_down[router] PASSED [ 80%]
test_scaling.py::test_scale_up_and_down[haproxy] PASSED [ 84%]
test_secondary_isolation_ist.py::test_secondary_isolation_ist_recovery[router] PASSED [ 88%]
test_secondary_isolation_ist.py::test_secondary_isolation_ist_recovery[haproxy] PASSED [ 92%]
test_secondary_isolation_sst.py::test_secondary_isolation_sst_recovery[router] PASSED [ 96%]
test_secondary_isolation_sst.py::test_secondary_isolation_sst_recovery[haproxy] PASSED [100%]

======================= 25 passed in 3846.83s (1:04:06) ========================