ci: cancel superseded PR workflow runs

[earayu]

What

Improve the high-frequency CI workflows:

• add GitHub Actions concurrency groups with cancel-in-progress: true to Unit-Test, Backend-Compat-Test, and the three E2E HTTP shape callers
• enable astral-sh/setup-uv caching keyed by uv.lock for the Python dependency-heavy Unit-Test and Backend-Compat-Test workflows
• remove the stale tests/integration/compat/test_graph_compat.py path from the compat workflow and Makefile target; that file has been replaced by test_lineage_graph_compat.py

Why

During rapid rebase/force-push cycles, older runs keep consuming runner time and can create stale signals. Dependency installs also repeat across runs even when uv.lock has not changed.

The compat workflow was also still invoking a deleted test file, so the graph compatibility matrix failed before exercising any backend. This PR restores the matrix to the existing compatibility test path.

Validation

• Ruby YAML parse for all touched workflow files
• make -n test-compat-graph
• git diff --check
• pre-commit make lint