Fall back to the text format for a non-numeric Accept version

[chrikrah]

choose_encoder compares the Accept version with parse_version(version) >= (1, 0, 0) (prometheus_client/exposition.py:375 and :384). With 1.0.0-rc.1, a quoted "1.0.0" or latest, that comparison raises TypeError and the scrape answers 500. This pull request treats a version with a non-numeric part as unsupported, so it falls through to the next media range and ends at text/plain; version=0.0.4, as version=1.0 already does.

Before, at 9cd073c:

$ python accept_version.py
'application/openmetrics-text; version=1.0.0-rc.1' -> raises TypeError '>=' not supported between instances of 'str' and 'int'
'application/openmetrics-text; version=latest' -> raises TypeError '>=' not supported between instances of 'str' and 'int'
'text/plain; version=1.0.0rc1' -> raises TypeError '>=' not supported between instances of 'str' and 'int'
'application/openmetrics-text; version=latest' client got HTTPError HTTP Error 500: Internal Server Error

Before #1136, packaging.Version raised InvalidVersion here for latest. Since #1136, a pre-release such as 1.0.0-rc.1 fails too.

Verification

# Python 3.14.7
$ python -m pytest -q tests
429 passed, 1 skipped, 3 subtests passed in 8.04s
# base 9cd073c: 428 passed, 1 skipped
$ git stash -- prometheus_client/exposition.py && python -m pytest -q tests/test_exposition.py   # fix reverted, test kept
3 failed, 98 passed, 1 skipped in 3.32s   # the 3 subtests of test_non_numeric_version, TypeError
$ flake8 prometheus_client/ tests/ && isort --check prometheus_client/ tests/   # flake8 7.4.1, isort 5.10.1
# not run: tox across Python versions; mypy 2.4.0 reports the same 2 errors in metrics.py on base and branch

No issue exists for this.

@csmarchbanks, should a quoted version="1.0.0" be unquoted and served as OpenMetrics 1.0.0? I can add that to this pull request.