Make dask[distributed] optional via covalent[dask], fix various unit and functional tests

[araghukas]

Require pip install covalent[dask] to install dask[distriburted] dependency for covalent, do not install dask[distributed] otherwise, with fallback to "local" executor.

Changes summary

Main point

• Make dask[distributed] and optional dependency via covalent[dask] extra
• Default executor changed from dask to local in defaults.py

Necessary fixes

• Dropped Python 3.10 support, added Python 3.13; updated test_matrix.json and CI workflows accordingly
• Fixed extract_graph to handle both "edges" (networkx ≥3.x) and "links" (older networkx) as the edge key; added lower bound on networkx version (would need this in many more places for Python 3.10 support)
• Fixed various 422 errors by setting additional Content-Type: application/json headers as necessary
• Moved locust out of tests/requirements.txt into the benchmark workflow only, to avoid a gevent conflict
• Various test fixes: renamed status → job_status, corrected mock paths, updated graph key references

---

• I have added the tests to cover my changes.
• I have updated the documentation and CHANGELOG accordingly.
• I have read the CONTRIBUTING document.

[araghukas]

Closing the other (duplicate) PR proved unnecessary. Instead I have removed Debian 12 / Python 3.10 / Dask and added Debian 12 / Python 3.13 / Dask in:

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