[ty] Add generic inference for dataclasses (#18443)

## Summary

An issue seen here https://github.com/astral-sh/ty/issues/500

The `__init__` method of dataclasses had no inherited generic context,
so we could not infer the type of an instance from a constructor call
with generics

## Test Plan

Add tests to classes.md` in generics folder

crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md

@@ -379,6 +379,21 @@ C[None](b"bytes")  # error: [no-matching-overload]
 C[None](12)
 ```
 
+### Synthesized methods with dataclasses
+
+```py
+from dataclasses import dataclass
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
+
+@dataclass
+class A(Generic[T]):
+    x: T
+
+reveal_type(A(x=1))  # revealed: A[int]
+```
+
 ## Generic subclass
 
 When a generic subclass fills its superclass's type parameter with one of its own, the actual types

crates/ty_python_semantic/resources/mdtest/generics/pep695/classes.md

@@ -354,6 +354,18 @@ C[None](b"bytes")  # error: [no-matching-overload]
 C[None](12)
 ```
 
+### Synthesized methods with dataclasses
+
+```py
+from dataclasses import dataclass
+
+@dataclass
+class A[T]:
+    x: T
+
+reveal_type(A(x=1))  # revealed: A[int]
+```
+
 ## Generic subclass
 
 When a generic subclass fills its superclass's type parameter with one of its own, the actual types

crates/ty_python_semantic/resources/mdtest/named_tuple.md

@@ -134,9 +134,7 @@ class Property[T](NamedTuple):
     name: str
     value: T
 
-# TODO: this should be supported (no error, revealed type of `Property[float]`)
+reveal_type(Property("height", 3.4))  # revealed: Property[float]
-# error: [invalid-argument-type]
-reveal_type(Property("height", 3.4))  # revealed: Property[Unknown]
 ```
 
 ## Attributes on `NamedTuple`

crates/ty_python_semantic/src/types/class.rs

@@ -1372,7 +1372,8 @@ impl<'db> ClassLiteral<'db> {
                 parameters.push(parameter);
             }
 
-            let signature = Signature::new(Parameters::new(parameters), Some(Type::none(db)));
+            let mut signature = Signature::new(Parameters::new(parameters), Some(Type::none(db)));
+            signature.inherited_generic_context = self.generic_context(db);
             Some(CallableType::function_like(db, signature))
         };