Add edit text support to stirling engine (#6245)

# Description of Changes
Hooks up the (alpha) PDF Editor backend to the AI engine Edit Agent via
an intermediary API which is easier for the agent to call. It suffers
from all the same issues that the PDF Editor does in actually editing
the text, but should also benefit from any fixes to that.

It also adds protection against the underlying tools misbehaving by
hanging, and fixes a hanging bug in the PDF Editor.

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Co-authored-by: EthanHealy01 <80844253+EthanHealy01@users.noreply.github.com>