[ty] Fix generic inference for non-dataclass inheriting from generic dataclass (#21159)

## Summary

Fixes https://github.com/astral-sh/ty/issues/1427

This PR fixes a regression introduced in alpha.24 where non-dataclass
children of generic dataclasses lost generic type parameter information
during `__init__` synthesis.

The issue occurred because when looking up inherited members in the MRO,
the child class's `inherited_generic_context` was correctly passed down,
but `own_synthesized_member()` (which synthesizes dataclass `__init__`
methods) didn't accept this parameter. It only used
`self.inherited_generic_context(db)`, which returned the parent's
context instead of the child's.

The fix threads the child's generic context through to the synthesis
logic, allowing proper generic type inference for inherited dataclass
constructors.

## Test Plan

- Added regression test for non-dataclass inheriting from generic
dataclass
- Verified the exact repro case from the issue now works
- All 277 mdtest tests passing
- Clippy clean
- Manually verified with Python runtime, mypy, and pyright - all accept
this code pattern

## Verification

Tested against multiple type checkers:
- ✅ Python runtime: Code works correctly
- ✅ mypy: No issues found
- ✅ pyright: 0 errors, 0 warnings
- ✅ ty alpha.23: Worked (before regression)
- ❌ ty alpha.24: Regression
- ✅ ty with this fix: Works correctly

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: David Peter <mail@david-peter.de>

crates/ty_python_semantic/src/types/class.rs

@@ -2176,7 +2176,8 @@ impl<'db> ClassLiteral<'db> {
         });
 
         if member.is_undefined() {
-            if let Some(synthesized_member) = self.own_synthesized_member(db, specialization, name)
+            if let Some(synthesized_member) =
+                self.own_synthesized_member(db, specialization, inherited_generic_context, name)
             {
                 return Member::definitely_declared(synthesized_member);
             }
@@ -2192,6 +2193,7 @@ impl<'db> ClassLiteral<'db> {
         self,
         db: &'db dyn Db,
         specialization: Option<Specialization<'db>>,
+        inherited_generic_context: Option<GenericContext<'db>>,
         name: &str,
     ) -> Option<Type<'db>> {
         let dataclass_params = self.dataclass_params(db);
@@ -2320,7 +2322,7 @@ impl<'db> ClassLiteral<'db> {
 
             let signature = match name {
                 "__new__" | "__init__" => Signature::new_generic(
-                    self.inherited_generic_context(db),
+                    inherited_generic_context.or_else(|| self.inherited_generic_context(db)),
                     Parameters::new(parameters),
                     return_ty,
                 ),
@@ -2702,7 +2704,7 @@ impl<'db> ClassLiteral<'db> {
         name: &str,
         policy: MemberLookupPolicy,
     ) -> PlaceAndQualifiers<'db> {
-        if let Some(member) = self.own_synthesized_member(db, specialization, name) {
+        if let Some(member) = self.own_synthesized_member(db, specialization, None, name) {
             Place::bound(member).into()
         } else {
             KnownClass::TypedDictFallback