test: add basic Git metadata samples

[vivekjm]

Related to #662.

This adds three small, clear-cut examples to the basic test corpus:

• a unified diff for a fictional weather-station project
• a .gitattributes file with common text, binary, and Linguist attributes
• a .gitmodules file with two fictional submodule entries

All three samples are original examples written from scratch for this pull request; no external source material was copied. I used an AI coding assistant while preparing and validating the contribution.

Tests:

• python/.venv/bin/magika --json tests_data/basic/diff/weather-station.patch tests_data/basic/gitattributes/repository.gitattributes tests_data/basic/gitmodules/project.gitmodules (all classified correctly with standard_v3_3, scores 0.999-1.000)
• python/.venv/bin/python python/scripts/run_quick_test_magika_module.py
• PATH="$PWD/python/.venv/bin:$PATH" python/.venv/bin/python python/scripts/run_quick_test_magika_cli.py --client-path python/.venv/bin/magika
• go test ./... (from go/)
• git diff --check