Task SDK: importable entry point for callback subprocesses so they can use fork+exec #73143

Body

#72164 lets the task process opt into fork+exec on Linux via [core] execute_tasks_new_python_interpreter, which closes the inherited-OpenSSL-lock hang in #71707 for tasks. Callbacks are still bare-forked: CallbackSubprocess.start passes a closure target, which WatchedSubprocess.start rejects under use_exec, so an on_failure_callback (or any other task callback) that makes a TLS call keeps the fork-inherited lock exposure after an operator flips the option.

Proposal: give callback subprocesses a named, importable target (the way tasks use _subprocess_main) so the same option covers them, and drop the "callbacks keep bare fork" caveat from the config text and the #72164 newsfragment once it lands.

Raised by @kaxil in #72164 (comment).

Are you willing to submit a PR?

Yes

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