Add flask test to click

[Rowlando13]

Using this workflow, add a CI job

$ git clone https://github.com/pallets/flask
$ cd flask
$ uv venv --python 3.14
$ source .venv/bin/activate
$ uv sync --all-extras
$ uv run --with "git+https://github.com/pallets/click.git@main" -- pytest

Also possibly at tox action to flask to run with click's latest

[kdeldycke]

Addressed in #3177