[ty] apply KW_ONLY sentinel only to local fields (#19986)

fix https://github.com/astral-sh/ty/issues/1047

## Summary

This PR fixes how `KW_ONLY` is applied in dataclasses. Previously, the
sentinel leaked into subclasses and incorrectly marked their fields as
keyword-only; now it only affects fields declared in the same class.

```py
from dataclasses import dataclass, KW_ONLY

@dataclass
class D:
    x: int
    _: KW_ONLY
    y: str

@dataclass
class E(D):
    z: bytes

# This should work: x=1 (positional), z=b"foo" (positional), y="foo" (keyword-only)
E(1, b"foo", y="foo")

reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
```

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

<!-- How was it tested? -->
mdtests

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

@@ -988,6 +988,28 @@ class D:  # error: [duplicate-kw-only]
         z: float
 ```
 
+`KW_ONLY` should only affect fields declared after it within the same class, not fields in
+subclasses:
+
+```py
+from dataclasses import dataclass, KW_ONLY
+
+@dataclass
+class D:
+    x: int
+    _: KW_ONLY
+    y: str
+
+@dataclass
+class E(D):
+    z: bytes
+
+# This should work: x=1 (positional), z=b"foo" (positional), y="foo" (keyword-only)
+E(1, b"foo", y="foo")
+
+reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
+```
+
 ## Other special cases
 
 ### `dataclasses.dataclass`

crates/ty_python_semantic/resources/mdtest/snapshots/dataclasses.md_-_Dataclasses_-_`dataclasses.KW_ONLY…_(dd1b8f2f71487f16).snap

@@ -49,6 +49,22 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.
 35 |         y: str
 36 |         _2: KW_ONLY
 37 |         z: float
+38 | from dataclasses import dataclass, KW_ONLY
+39 | 
+40 | @dataclass
+41 | class D:
+42 |     x: int
+43 |     _: KW_ONLY
+44 |     y: str
+45 | 
+46 | @dataclass
+47 | class E(D):
+48 |     z: bytes
+49 | 
+50 | # This should work: x=1 (positional), z=b"foo" (positional), y="foo" (keyword-only)
+51 | E(1, b"foo", y="foo")
+52 | 
+53 | reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
 ```
 
 # Diagnostics
@@ -141,3 +157,15 @@ info: `KW_ONLY` fields: `_1`, `_2`
 info: rule `duplicate-kw-only` is enabled by default
 
 ```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:53:13
+   |
+51 | E(1, b"foo", y="foo")
+52 |
+53 | reveal_type(E.__init__)  # revealed: (self: E, x: int, z: bytes, *, y: str) -> None
+   |             ^^^^^^^^^^ `(self: E, x: int, z: bytes, *, y: str) -> None`
+   |
+
+```