Run task callbacks in fresh interpreters

[keith991001]

Reviewed by @keith991001: this change addresses the callback-specific
fork+exec gap described in #73143.

Task callbacks used a closure as their subprocess target, so they could not
opt into WatchedSubprocess's fork+exec path and remained exposed to locks
inherited from supervisor threads.

This change:

• adds an importable callback child entry point and sends startup data over
  the existing supervisor socket;
• applies execute_tasks_new_python_interpreter to callback subprocesses;
• adds a real fork+exec callback regression test and focused unit coverage;
• updates the configuration documentation and the existing significant
  newsfragment.

Tests:

• uv run --no-sync --project task-sdk pytest task-sdk/tests/task_sdk/execution_time/test_callback_supervisor.py -q
• uv run --no-sync prek run mypy-task-sdk --all-files
• Ruff formatting and lint checks
• Applicable pre-commit and manual hooks

closes: #73143

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Was generative AI tooling used to co-author this PR?

• Yes — Codex (GPT-5)

Generated-by: Codex (GPT-5) following the guidelines

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Drafted-by: Codex (GPT-5); reviewed by @keith991001 before posting