[ty] Subscript assignment diagnostics follow-up (#21452)

## Summary

Follow up from https://github.com/astral-sh/ruff/pull/21411. Again,
there are more things that could be improved here (like the diagnostics
for `lists`, or extending what we have for `dict` to `OrderedDict` etc),
but that will have to be postponed.

crates/ty_python_semantic/resources/mdtest/subscript/assignment_diagnostics.md

@@ -4,6 +4,15 @@
 
 ## Invalid value type
 
+### For a `list`
+
+```py
+numbers: list[int] = []
+numbers[0] = "three"  # error: [invalid-assignment]
+```
+
+### For a `dict`
+
 ```py
 config: dict[str, int] = {}
 config["retries"] = "three"  # error: [invalid-assignment]
@@ -11,6 +20,15 @@ config["retries"] = "three"  # error: [invalid-assignment]
 
 ## Invalid key type
 
+### For a `list`
+
+```py
+numbers: list[int] = []
+numbers["zero"] = 3  # error: [invalid-assignment]
+```
+
+### For a `dict`
+
 ```py
 config: dict[str, int] = {}
 config[0] = 3  # error: [invalid-assignment]

crates/ty_python_semantic/resources/mdtest/subscript/instance.md

@@ -110,6 +110,6 @@ class Identity:
         pass
 
 a = Identity()
-# error: [invalid-assignment] "Method `__setitem__` of type `bound method Identity.__setitem__(index: int, value: int) -> None` cannot be called with a key of type `Literal["a"]` and a value of type `Literal[0]` on object of type `Identity`"
+# error: [invalid-assignment] "Invalid subscript assignment with key of type `Literal["a"]` and value of type `Literal[0]` on object of type `Identity`"
 a["a"] = 0
 ```