[ty] Type-context aware literal promotion (#20776)

## Summary

Avoid literal promotion when a literal type annotation is provided, e.g.,
```py
x: list[Literal[1]] = [1]
```

Resolves https://github.com/astral-sh/ty/issues/1198. This does not fix
issue https://github.com/astral-sh/ty/issues/1284, but it does make it
more relevant because after this change, it is possible to directly
instantiate a generic type with a literal specialization.

crates/ty_python_semantic/resources/mdtest/assignment/annotations.md

@@ -130,13 +130,9 @@ type IntList = list[int]
 m: IntList = [1, 2, 3]
 reveal_type(m)  # revealed: list[int]
 
-# TODO: this should type-check and avoid literal promotion
-# error: [invalid-assignment] "Object of type `list[Unknown | int]` is not assignable to `list[Literal[1, 2, 3]]`"
 n: list[typing.Literal[1, 2, 3]] = [1, 2, 3]
 reveal_type(n)  # revealed: list[Literal[1, 2, 3]]
 
-# TODO: this should type-check and avoid literal promotion
-# error: [invalid-assignment] "Object of type `list[Unknown | str]` is not assignable to `list[LiteralString]`"
 o: list[typing.LiteralString] = ["a", "b", "c"]
 reveal_type(o)  # revealed: list[LiteralString]
 
@@ -160,6 +156,81 @@ a: list[str] = [1, 2, 3]
 b: set[int] = {1, 2, "3"}
 ```
 
+## Literal annnotations are respected
+
+```toml
+[environment]
+python-version = "3.12"
+```
+
+```py
+from enum import Enum
+from typing_extensions import Literal, LiteralString
+
+a: list[Literal[1]] = [1]
+reveal_type(a)  # revealed: list[Literal[1]]
+
+b: list[Literal[True]] = [True]
+reveal_type(b)  # revealed: list[Literal[True]]
+
+c: list[Literal["a"]] = ["a"]
+reveal_type(c)  # revealed: list[Literal["a"]]
+
+d: list[LiteralString] = ["a", "b", "c"]
+reveal_type(d)  # revealed: list[LiteralString]
+
+e: list[list[Literal[1]]] = [[1]]
+reveal_type(e)  # revealed: list[list[Literal[1]]]
+
+class Color(Enum):
+    RED = "red"
+
+f: dict[list[Literal[1]], list[Literal[Color.RED]]] = {[1]: [Color.RED, Color.RED]}
+reveal_type(f)  # revealed: dict[list[Literal[1]], list[Literal[Color.RED]]]
+
+class X[T]:
+    def __init__(self, value: T): ...
+
+g: X[Literal[1]] = X(1)
+reveal_type(g)  # revealed: X[Literal[1]]
+
+h: X[int] = X(1)
+reveal_type(h)  # revealed: X[int]
+
+i: dict[list[X[Literal[1]]], set[Literal[b"a"]]] = {[X(1)]: {b"a"}}
+reveal_type(i)  # revealed: dict[list[X[Literal[1]]], set[Literal[b"a"]]]
+
+j: list[Literal[1, 2, 3]] = [1, 2, 3]
+reveal_type(j)  # revealed: list[Literal[1, 2, 3]]
+
+k: list[Literal[1] | Literal[2] | Literal[3]] = [1, 2, 3]
+reveal_type(k)  # revealed: list[Literal[1, 2, 3]]
+
+type Y[T] = list[T]
+
+l: Y[Y[Literal[1]]] = [[1]]
+reveal_type(l)  # revealed: list[list[Literal[1]]]
+
+m: list[tuple[Literal[1], Literal[2], Literal[3]]] = [(1, 2, 3)]
+reveal_type(m)  # revealed: list[tuple[Literal[1], Literal[2], Literal[3]]]
+
+n: list[tuple[int, str, int]] = [(1, "2", 3), (4, "5", 6)]
+reveal_type(n)  # revealed: list[tuple[int, str, int]]
+
+o: list[tuple[Literal[1], ...]] = [(1, 1, 1)]
+reveal_type(o)  # revealed: list[tuple[Literal[1], ...]]
+
+p: list[tuple[int, ...]] = [(1, 1, 1)]
+reveal_type(p)  # revealed: list[tuple[int, ...]]
+
+# literal promotion occurs based on assignability, an exact match is not required
+q: list[int | Literal[1]] = [1]
+reveal_type(q)  # revealed: list[int]
+
+r: list[Literal[1, 2, 3, 4]] = [1, 2]
+reveal_type(r)  # revealed: list[Literal[1, 2, 3, 4]]
+```
+
 ## PEP-604 annotations are supported
 
 ```py

crates/ty_python_semantic/resources/mdtest/type_compendium/tuple.md

@@ -525,10 +525,6 @@ from typing import Literal
 reveal_type(list((1, 2, 3)))  # revealed: list[int]
 reveal_type(list(((1, 2, 3),)))  # revealed: list[tuple[int, int, int]]
 
-# TODO: we could bidirectionally infer that the user does not want literals to be promoted here,
-# and avoid this diagnostic
-#
-# error: [invalid-assignment] "`list[int]` is not assignable to `list[Literal[1, 2, 3]]`"
 x: list[Literal[1, 2, 3]] = list((1, 2, 3))
 reveal_type(x)  # revealed: list[Literal[1, 2, 3]]
 ```