[DKP-2535] Honour context if present for default client, add contexts support

[ebriney]

Summary of changes:

• docker/context/api.py — new ContextAPI.kwargs_from_context(name=None, environment=None) helper that resolves a Docker CLI context (honouring DOCKER_CONTEXT, currentContext in ~/.docker/config.json, then default) and returns {base_url, tls} ready
  for APIClient.
• docker/client.py —
  • DockerClient.from_env() now falls back to the current context when DOCKER_HOST is unset, so on a Docker Desktop machine docker.from_env() automatically talks to desktop-linux. Opt out with use_context=False.
  • New DockerClient.from_context(name=None, ...) classmethod for explicit context selection, also exported as docker.from_context.
• docker/init.py — re-exports from_context.
• tests/unit/client_test.py —
  • Pinned the two from_env pool-size tests with use_context=False so they stay deterministic regardless of the developer's current context.
  • New FromContextTest class covering: current-context fallback, DOCKER_HOST precedence, opt-out via use_context=False, from_context(name=...), unknown-context safety, and default-context resolution.

All 14 new/adjusted tests pass; the remaining pytest tests/unit/ failures are pre-existing on main (verified by stashing and re-running). The behavior matches the Docker CLI: DOCKER_HOST wins, then DOCKER_CONTEXT, then currentContext, then default.