[ty] Prevent type that is written with a code span from being incorrectly re-wrapped during docstring Markdown rendering (#26723)
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## Summary
This does two things:
1. (abe15fe4) It refactors and consolidates the helpers that we use for
parsing runs of backticks (which denote the opening and closing of
Markdown fences). This lays a foundation for [rendering Google- and
NumPy- style
docstrings](https://github.com/astral-sh/ty/issues/1667)[^1].
2. (3acd8a4d) It fixes a bug in Markdown rendering for reStructuredText
docstrings. Previously the parameter type in the following docstring
would incorrectly be rendered as a code span containing literal
backticks (i.e., `` `str` ``), because we would re-wrap the type in
another set of backticks:
```text
:param value: The value.
:type value: `str`
```
With this change, we now output a type that is already wrapped verbatim
(so that the above example renders as `str`).
[^1]: Google- and NumPy- style docstrings may contain types that are
already wrapped in code spans:
```text
Args:
value (`str`): The value.
```
```text
Parameters
----------
value : `str`
The value.
```
## Test Plan
See included tests.
<!-- How was it tested? -->
ruff
39281658 - [ty] Prevent type that is written with a code span from being incorrectly re-wrapped during docstring Markdown rendering (#26723)
