[ty] Conditionally defined dataclass fields (#19197)

## Summary

Fixes a bug where conditionally defined dataclass fields were previously
ignored.

Thanks to @lipefree for reporting this.

## Test Plan

New Markdown tests

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

@@ -558,6 +558,50 @@ class C(Base):
 reveal_type(C.__init__)  # revealed: (self: C, x: int = Literal[15], y: int = Literal[0], z: int = Literal[10]) -> None
 ```
 
+## Conditionally defined fields
+
+### Statically known conditions
+
+Fields that are defined in always-reachable branches are always present in the synthesized
+`__init__` method. Fields that are defined in never-reachable branches are not present:
+
+```py
+from dataclasses import dataclass
+
+@dataclass
+class C:
+    normal: int
+
+    if 1 + 2 == 3:
+        always_present: str
+
+    if 1 + 2 == 4:
+        never_present: bool
+
+reveal_type(C.__init__)  # revealed: (self: C, normal: int, always_present: str) -> None
+```
+
+### Dynamic conditions
+
+If a field is conditionally defined, we currently assume that it is always present. A more complex
+alternative here would be to synthesized a union of all possible `__init__` signatures:
+
+```py
+from dataclasses import dataclass
+
+def flag() -> bool:
+    return True
+
+@dataclass
+class C:
+    normal: int
+
+    if flag():
+        conditionally_present: str
+
+reveal_type(C.__init__)  # revealed: (self: C, normal: int, conditionally_present: str) -> None
+```
+
 ## Generic dataclasses
 
 ```toml
@@ -789,6 +833,23 @@ class Fails:  # error: [duplicate-kw-only]
 reveal_type(Fails.__init__)  # revealed: (self: Fails, a: int, *, c: str, e: bytes) -> None
 ```
 
+This also works if `KW_ONLY` is used in a conditional branch:
+
+```py
+def flag() -> bool:
+    return True
+
+@dataclass
+class D:  # error: [duplicate-kw-only]
+    x: int
+    _1: KW_ONLY
+
+    if flag():
+        y: str
+        _2: KW_ONLY
+        z: float
+```
+
 ## Other special cases
 
 ### `dataclasses.dataclass`

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

@@ -37,6 +37,18 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.
 23 |     e: bytes
 24 | 
 25 | reveal_type(Fails.__init__)  # revealed: (self: Fails, a: int, *, c: str, e: bytes) -> None
+26 | def flag() -> bool:
+27 |     return True
+28 | 
+29 | @dataclass
+30 | class D:  # error: [duplicate-kw-only]
+31 |     x: int
+32 |     _1: KW_ONLY
+33 | 
+34 |     if flag():
+35 |         y: str
+36 |         _2: KW_ONLY
+37 |         z: float
 ```
 
 # Diagnostics
@@ -109,6 +121,23 @@ info[revealed-type]: Revealed type
 24 |
 25 | reveal_type(Fails.__init__)  # revealed: (self: Fails, a: int, *, c: str, e: bytes) -> None
    |             ^^^^^^^^^^^^^^ `(self: Fails, a: int, *, c: str, e: bytes) -> None`
+26 | def flag() -> bool:
+27 |     return True
    |
 
 ```
+
+```
+error[duplicate-kw-only]: Dataclass has more than one field annotated with `KW_ONLY`
+  --> src/mdtest_snippet.py:30:7
+   |
+29 | @dataclass
+30 | class D:  # error: [duplicate-kw-only]
+   |       ^
+31 |     x: int
+32 |     _1: KW_ONLY
+   |
+info: `KW_ONLY` fields: `_1`, `_2`
+info: rule `duplicate-kw-only` is enabled by default
+
+```

crates/ty_python_semantic/src/types/class.rs

@@ -1650,7 +1650,7 @@ impl<'db> ClassLiteral<'db> {
             if !declarations
                 .clone()
                 .all(|DeclarationWithConstraint { declaration, .. }| {
-                    declaration.is_defined_and(|declaration| {
+                    declaration.is_undefined_or(|declaration| {
                         matches!(
                             declaration.kind(db),
                             DefinitionKind::AnnotatedAssignment(..)