Read the cluster's distribution version instead of returning a constant

[serhiy-bzhezytskyy]

Description

cluster_distribution_version is documented as "Attempt to get the cluster's distribution version", but it created a client and then ignored it (solrorbit/builder/builder.py:272):

client_instance = client_factory(hosts, client_options).create()
if isinstance(client_instance, client.SolrClient):
    return "9.10.1"
return None

Every externally provisioned run that does not pass --distribution-version was therefore told the cluster is 9.10.1, whatever it actually is.

That value is not informational — it selects the workload branch. WorkloadRepository.update feeds it to versions.best_match (solrorbit/utils/repo.py:71):

versions.best_match(["main", "9", "10"], "9.10.1")  -> '9'
versions.best_match(["main", "9", "10"], "10.0.0")  -> '10'

CONTRIBUTING.md in solr-orbit-workloads documents that selection as intended — "solr-orbit automatically selects the workload branch that matches the Solr major version being tested" — so once a 10 branch exists there, a Solr 10 cluster gets benchmarked with the Solr 9 workloads and the run succeeds silently.

This calls the client's existing get_version(), which reads lucene.solr-spec-version from GET /api/node/system. When the cluster cannot be read it raises SystemSetupError naming --distribution-version rather than substituting a constant, since a wrong value is worse than a stop.

Verified against an unmodified solr:latest container (Solr 10.0.0, solr-impl 10.0.0 6c6c48a6f78, Lucene 10.3.2):

cluster_distribution_version()  ->  '10.0.0'      (was '9.10.1')
  best_match(['main','9','10'], '10.0.0') -> '10'  (was '9')

unreachable host -> SystemSetupError: Could not determine the distribution version of the
  cluster at [{'host': 'localhost', 'port': 9999}]. Specify it with --distribution-version.
  Cause: HTTPConnectionPool(...)

One deliberate consequence: the minimum_solr_version check in test_run_orchestrator.setup() was inert, because it compared a constant against a constant. It now compares against the real cluster. min-version.txt is 9.0.0, so 9.x and 10.x are unaffected and the only newly-rejected clusters are Solr 8 and older, which are already below the documented minimum.

Issues Resolved

Fixes #70

Testing

• New functionality includes testing

Four tests in tests/builder/mechanic_test.py: the version is read from the cluster, the resulting best_match picks the cluster's own major, an unreachable cluster raises SystemSetupError naming the flag, and a non-Solr client still returns None.

Three of the four fail without the change ('10.0.0' != '9.10.1', '10' != '9', SystemSetupError not raised); the fourth is a regression guard on existing behaviour and passes either way.

ruff check . clean; full unit suite 1108 passed / 5 skipped. make it was not run.