fix: allow editable namespace siblings with Python 3 wheels

[astrogilda]

Legacy namespace registration prevents a separately installed editable package under google from importing with python -I. This removes that registration and adds a Python 3.10–3.14 job that checks a real editable namespace sibling in both installation orders, with ordinary and isolated Python.

The job also catches Python 2 wheel tags and hash-dependent requirement ordering. The change emits Python 3 tags, preserves them after generation, sorts the existing requirements, and checks full wheel byte equality under two hash seeds. The contribution guide now matches the declared Nox sessions and survives generation.

Validation on Linux:

• The exact workflow passed on all five Python versions, including both installation orders and both hash seeds.
• The installed wheels preserve all 1,691 provider Python files. Unchanged ADK 650394f imports Workflow with four provider APIs in ordinary and isolated Python.
• All 1,729 other wheel members preserve their bytes. Metadata preserves every header and the body. Bootstrap, generated tags, malformed-wheel refusals, guide parsing and applicable hooks pass.

Validation covers local installation and imports. Windows and cloud service calls remain untested.