[ty] TypedDict: Add support for typing.ReadOnly (#20241)

## Summary

Add support for `typing.ReadOnly` as a type qualifier to mark
`TypedDict` fields as being read-only. If you try to mutate them, you
get a new diagnostic:

<img width="787" height="234" alt="image"
src="https://github.com/user-attachments/assets/f62fddf9-4961-4bcd-ad1c-747043ebe5ff"
/>


## Test Plan

* New Markdown tests
* The typing conformance changes are all correct. There are some false
negatives, but those are related to the missing support for the
functional form of `TypedDict`, or to overriding of fields via
inheritance. Both of these topics are tracked in
https://github.com/astral-sh/ty/issues/154

crates/ty_python_semantic/resources/mdtest/snapshots/typed_dict.md_-_`TypedDict`_-_Diagnostics_(e5289abf5c570c29).snap

@@ -37,6 +37,14 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/typed_dict.md
 23 | 
 24 | def write_to_non_literal_string_key(person: Person, str_key: str):
 25 |     person[str_key] = "Alice"  # error: [invalid-key]
+26 | from typing_extensions import ReadOnly
+27 | 
+28 | class Employee(TypedDict):
+29 |     id: ReadOnly[int]
+30 |     name: str
+31 | 
+32 | def write_to_readonly_key(employee: Employee):
+33 |     employee["id"] = 42  # error: [invalid-assignment]
 ```
 
 # Diagnostics
@@ -127,7 +135,22 @@ error[invalid-key]: Cannot access `Person` with a key of type `str`. Only string
 24 | def write_to_non_literal_string_key(person: Person, str_key: str):
 25 |     person[str_key] = "Alice"  # error: [invalid-key]
    |            ^^^^^^^
+26 | from typing_extensions import ReadOnly
    |
 info: rule `invalid-key` is enabled by default
 
 ```
+
+```
+error[invalid-assignment]: Can not assign to key "id" on TypedDict `Employee`
+  --> src/mdtest_snippet.py:33:5
+   |
+32 | def write_to_readonly_key(employee: Employee):
+33 |     employee["id"] = 42  # error: [invalid-assignment]
+   |     -------- ^^^^ key is marked read-only
+   |     |
+   |     TypedDict `Employee`
+   |
+info: rule `invalid-assignment` is enabled by default
+
+```

crates/ty_python_semantic/resources/mdtest/typed_dict.md

@@ -444,8 +444,7 @@ def _(person: Person, unknown_key: Any):
 
 ## `ReadOnly`
 
-`ReadOnly` is not supported yet, but this test makes sure that we do not emit any false positive
-diagnostics:
+Assignments to keys that are marked `ReadOnly` will produce an error:
 
 ```py
 from typing_extensions import TypedDict, ReadOnly, Required
@@ -458,10 +457,26 @@ class Person(TypedDict, total=False):
 alice: Person = {"id": 1, "name": "Alice", "age": 30}
 alice["age"] = 31  # okay
 
-# TODO: this should be an error
+# error: [invalid-assignment] "Can not assign to key "id" on TypedDict `Person`: key is marked read-only"
 alice["id"] = 2
 ```
 
+This also works if all fields on a `TypedDict` are `ReadOnly`, in which case we synthesize a
+`__setitem__` method with a `key` type of `Never`:
+
+```py
+class Config(TypedDict):
+    host: ReadOnly[str]
+    port: ReadOnly[int]
+
+config: Config = {"host": "localhost", "port": 8080}
+
+# error: [invalid-assignment] "Can not assign to key "host" on TypedDict `Config`: key is marked read-only"
+config["host"] = "127.0.0.1"
+# error: [invalid-assignment] "Can not assign to key "port" on TypedDict `Config`: key is marked read-only"
+config["port"] = 80
+```
+
 ## Methods on `TypedDict`
 
 ```py
@@ -846,6 +861,19 @@ def write_to_non_literal_string_key(person: Person, str_key: str):
     person[str_key] = "Alice"  # error: [invalid-key]
 ```
 
+Assignment to `ReadOnly` keys:
+
+```py
+from typing_extensions import ReadOnly
+
+class Employee(TypedDict):
+    id: ReadOnly[int]
+    name: str
+
+def write_to_readonly_key(employee: Employee):
+    employee["id"] = 42  # error: [invalid-assignment]
+```
+
 ## Import aliases
 
 `TypedDict` can be imported with aliases and should work correctly: