[ty] Add tests for dataclass fields annotated with Final (#19202)

## Summary

Adds some tests for dataclass fields that are annotated with `Final`
(see comment
[here](https://github.com/astral-sh/ruff/pull/15768#issuecomment-3044737645)).
Turns out that nothing is needed here, everything already works as
expected (apart from the fact that we can assign to `Final` fields,
which is tracked in https://github.com/astral-sh/ty/issues/158

## Test Plan

New Markdown tests

crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md

@@ -444,6 +444,33 @@ To do
 
 To do
 
+## `Final` fields
+
+Dataclass fields can be annotated with `Final`, which means that the field cannot be reassigned
+after the instance is created. Fields that are additionally annotated with `ClassVar` are not part
+of the `__init__` signature.
+
+```py
+from dataclasses import dataclass
+from typing import Final, ClassVar
+
+@dataclass
+class C:
+    # a `Final` annotation without a right-hand side is not allowed in normal classes,
+    # but valid for dataclasses. The field will be initialized in the synthesized
+    # `__init__` method
+    instance_variable_no_default: Final[int]
+    instance_variable: Final[int] = 1
+    class_variable1: ClassVar[Final[int]] = 1
+    class_variable2: ClassVar[Final[int]] = 1
+
+reveal_type(C.__init__)  # revealed: (self: C, instance_variable_no_default: int, instance_variable: int = Literal[1]) -> None
+
+c = C(1)
+# TODO: this should be an error
+c.instance_variable = 2
+```
+
 ## Inheritance
 
 ### Normal class inheriting from a dataclass

crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md

@@ -162,6 +162,10 @@ from typing import Final
 
 # TODO: This should be an error
 NO_RHS: Final
+
+class C:
+    # TODO: This should be an error
+    NO_RHS: Final
 ```
 
 [`typing.final`]: https://docs.python.org/3/library/typing.html#typing.Final