Refactor semantic syntax error scope handling (#17314)

## Summary

Based on the discussion in
https://github.com/astral-sh/ruff/pull/17298#discussion_r2033975460, we
decided to move the scope handling out of the `SemanticSyntaxChecker`
and into the `SemanticSyntaxContext` trait. This PR implements that
refactor by:

- Reverting all of the `Checkpoint` and `in_async_context` code in the
`SemanticSyntaxChecker`
- Adding four new methods to the `SemanticSyntaxContext` trait
- `in_async_context`: matches `SemanticModel::in_async_context` and only
detects the nearest enclosing function
- `in_sync_comprehension`: uses the new `is_async` tracking on
`Generator` scopes to detect any enclosing sync comprehension
  - `in_module_scope`: reports whether we're at the top-level scope
  - `in_notebook`: reports whether we're in a Jupyter notebook
- In-lining the `TestContext` directly into the
`SemanticSyntaxCheckerVisitor`
- This allows modifying the context as the visitor traverses the AST,
which wasn't possible before

One potential question here is "why not add a single method returning a
`Scope` or `Scopes` to the context?" The main reason is that the `Scope`
type is defined in the `ruff_python_semantic` crate, which is not
currently a dependency of the parser. It also doesn't appear to be used
in red-knot. So it seemed best to use these more granular methods
instead of trying to access `Scope` in `ruff_python_parser` (and
red-knot).

## Test Plan

Existing parser and linter tests.

crates/ruff_python_parser/resources/inline/ok/nested_async_comprehension_py310.py

@@ -1,9 +1,7 @@
 # parse_options: {"target-version": "3.10"}
-# this case fails if exit_expr doesn't run
 async def f():
     [_ for n in range(3)]
     [_ async for n in range(3)]
-# and this fails without exit_stmt
 async def f():
     def g(): ...
     [_ async for n in range(3)]