[ty] Recognize that dedent ends doctest block when parsing docstrings for signature help (#26552)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
This fixes a small bug in the logic for recognizing the end of a doctest
in docstring. The immediate impact is low because the shape that exposes
the bug is uncommon in reStructuredText, but the shape is more common in
[the Google and NumPy docstring formats that we hope to render as
Markdown soon](https://github.com/astral-sh/ty/issues/1667).
## Test Plan
See included test.
<!-- How was it tested? -->
ruff
001298b4 - [ty] Recognize that dedent ends doctest block when parsing docstrings for signature help (#26552)
