Tests against a matrix of supported flagsmith sdk versions ?

CI currently tests the locked flagsmith version (`6.2.0`) across all Python versions, plus a single job pinning the floor (`5.5.0`).

Add a flagsmith version axis to the pytest workflow, analogous to the matrix in the Python SDK repo, e.g. floor (`5.5.0`), each minor up to latest, and the lock, so signature drift in any supported version fails CI instead of surfacing at runtime.

Version-gated tests (engine reasons, >=6.2) already skip below their required version, so the matrix needs no test changes.

Start with the pytest workflow and compare its current Python/version jobs with the matrix used in the Python SDK repository. Add the floor version 5.5.0, each supported minor through the latest version, and the locked version 6.2.0 as the Flagsmith axis. The existing version-gated tests should remain unchanged, and the workflow should pass across every matrix entry.

Written by the indexing model from the issue text.