Dynamically discover micropython/unix Docker versions for test_typing_runtime

- [x] Add `get_unix_docker_versions()` function that fetches available `micropython/unix` Docker image tags from Docker Hub - [x] Filter to proper version tags (e.g. `v1.20.0` and above) using a minimum version filter and regex validation - [x] Include `_KNOWN_UNIX_VERSIONS` fallback list for when Docker Hub is unreachable - [x] Log exception details when Docker Hub fetch fails (with `exc_info=True`) - [x] Move `json` import to top-level with other standard library imports - [x] Replace hardcoded `mp_version` list in `test_typing_runtime` parametrize with `get_unix_docker_versions()` - [x] Remove unused `version` parameter (was hardcoded `"preview"`) from `test_typing_runtime` — it was inherited from `test_snippets.py` pattern but neither the test body nor the `copy_mpy_typings_fx` fixture used it; it was polluting every test ID with `-preview-` <!-- START COPILOT CODING AGENT SUFFIX --> <!-- START COPILOT ORIGINAL PROMPT --> <details> <summary>Original prompt</summary> > the test tests/quality_tests/test_typings.py::test_typing_runtime > > appears to only test against an older number of versions > > explore how all MicroPython version can be tested </details> <!-- START COPILOT CODING AGENT TIPS --> --- 💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more [Copilot coding agent tips](https://gh.io/copilot-coding-agent-tips) in the docs.