Test E2E - add tests for the distance tool (0.10.x) #40

TL;DR: distance (new in 0.10.x) is neither referenced nor tested.

• Add TOOL_DISTANCE to config/constants.py and to EXPECTED_TOOLS when GEOCONTEXT_DEV=1 (also fix the "10 outils" docstring in test_tools_discovery.py).
• Add a simple chaining test, e.g. "Quelle distance à pied entre la gare de Lyon et la tour Eiffel ?" → geocode + distance (profile=pedestrian).
• Port the scenario from geocontext PR Add time metric and car and pedestrian profiles to DistanceTool geocontext#207 (from New test to port (done with test_chaining_distance_piscine_caen.py but maybe too complex) #38): "Quel est le temps de marche exact entre le cinéma LUX, situé au sud-est de Caen, et la piscine la plus proche ?" → geocode, gpf_search_types, gpf_describe_type, gpf_get_features, distance; answer contains "Sivom", "Mondeville" and a walking time in 25–35 min.
• Document the new test(s) in English in the tests tables of README.md (File, Tool / Tool chain, Description), see Mixing french and english in documentation #37.

Tools reference: https://github.com/ignfab/geocontext/blob/main/docs/mcp-tools.md