Run CLI tests in CI

[ppolewicz]

Supersedes #260.

This revives the original idea on top of the current CI layout:

• add a dedicated cli-compat workflow job that checks out Backblaze/B2_Command_Line_Tool
• install this SDK checkout into the CLI test environment via INSTALL_SDK_FROM
• run CLI unit tests across a focused compatibility matrix
• run non-secret CLI integration tests across that matrix
• run secret-backed CLI integration on a single Linux / Python 3.14 lane to keep CI load bounded

Validation run locally:

• uv run nox -vs towncrier_check
• CLI unit tests against this checkout on Python 3.12
• CLI non-secret integration tests against this checkout on Python 3.12 (b2v3 and b2v4)