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Fix interactive processes hanging on terminal read (#23659)

PR #23466 changed the way processes were spawned through ManagedChild.
Prior to that change, a spawned process would be created in a new
process group, in a new session, but that mechanism precluded a more
efficient process-spawning path. Ostensibly (per change history, local
comments, test coverage), the spawning of the child in a new session was
merely an implementation quirk of ensuring the child was spawned in a
new process group, to make it safe to kill that child's process group
later; the creation of the new session itself was incidental.
Eliminating the constraint of also creating the child in a new session
permitted the more efficient spawning path in #23466.

However, for spawned _interactive_ processes specifically there _is_ a
dependency on spawning in a new session, as revealed by #23628: when an
interactive process reasonably tries to read terminal input, and because
it wasn't spawned in a new session but instead into a (new) background
process group of the same session as the controlling terminal, it will
be signalled with SIGTTIN and suspended.

This change preserves the "fast-path" spawning mechanism for
non-interactive processes, but restores the spawning mechanism replaced
in #23466 for just interactive processes.

Also adds a test to protect against regressing this behavior again.

Notice: An LLM assisted identifying the callpath through which
interactive processes were ultimately spawned by ManagedChild, and wrote
an initial skeleton for this change and for a unittest. This code was
reviewed and edited by a human. All comments were written (or moved) by
a human. Style and naming choices were made by a human.

Fixes #23628

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