Add Python 3.14 to the CI test matrix #832

Python 3.14 was released in October 2025. rsconnect-python's pyproject.toml declares requires-python = ">=3.8" with no upper bound, so 3.14 is technically allowed. But the CI matrix in .github/workflows/main.yml only tests 3.8 through 3.13. There is no explicit test coverage for 3.14.

I checked from posit-cli, which depends on rsconnect-python. A quick manual check on 3.14.3 looks good:

$ uv run --python 3.14 rsconnect version
1.30.0
$ uv run --python 3.14 python -c "
from rsconnect.main import cli
from rsconnect.api import RSConnectClient, RSConnectExecutor, RSConnectException
from rsconnect.http_support import HTTPResponse
print('ok')
"
ok

This is not a full test run of rsconnect-python's own suite, just an import and CLI smoke check. Please add 3.14 to the CI test matrix and confirm the full suite passes, or narrow requires-python if it does not.