fix(sdk/python): allow skipping container IP detection

[mikemikimike]

Summary

• add an opt-out for container and public IP discovery in restricted deployments
• preserve the existing detection order and default behavior
• add a deterministic regression test proving no network request is made when disabled

Closes #624

Validation

• uv run --frozen pytest tests/test_agent_networking.py -q (15 passed)
• uv run --frozen ruff check agentfield/agent.py tests/test_agent_networking.py
• uv run --frozen ruff format --check agentfield/agent.py tests/test_agent_networking.py
• git diff --check
• Full suite: 11 failures unrelated to this change (Windows shell/encoding assumptions in test_harness_packaging.py and test_run_cli_env.py); remaining tests passed, 6 skipped.

[AbirAbbas]

Thanks for jumping on this — and sorry for the collision: #969 went up within seconds of this one and covers the same opt-out plus the case pocesar actually reported in #624 (the probe firing even though a callback URL is already configured — #969 skips detection automatically whenever callback_url / AGENT_CALLBACK_URL is set, and adds AGENTFIELD_DISABLE_IP_DETECTION for containers with no callback URL at all), with a tripwire test that requests.get is never reached and a docs entry. Since #969 is already reviewed and queued, I'm closing this one rather than carry two fixes for one issue. One note for next time: about two thirds of this diff is ruff format reflow of untouched code in agent.py — the repo has pre-existing drift there and CI doesn't gate formatting, so keeping diffs to the hunks you mean to change makes review much faster. Your #947 is in the merge queue, by the way — thanks for that one.