[ty] Implicit instance attributes declared Final (#19462)

## Summary

Adds proper type inference for implicit instance attributes that are
declared with a "bare" `Final` and adds `invalid-assignment` diagnostics
for all implicit instance attributes that are declared `Final` or
`Final[…]`.

## Test Plan

New and updated MD tests.

## Ecosystem analysis

```diff
pytest (https://github.com/pytest-dev/pytest)
+ error[invalid-return-type] src/_pytest/fixtures.py:1662:24: Return type does not match returned value: expected `Scope`, found `Scope | (Unknown & ~None & ~((...) -> object) & ~str) | (((str, Config, /) -> Unknown) & ~((...) -> object) & ~str) | (Unknown & ~str)
```

The definition of the `scope` attribute is [here](

5f99385635/src/_pytest/fixtures.py (L1020-L1028)).
Looks like this is a new false positive due to missing `TypeAlias`
support that is surfaced here because we now infer a more precise type
for `FixtureDef._scope`.

crates/ty_python_semantic/src/semantic_index/builder.rs

@@ -1421,6 +1421,13 @@ impl<'ast> Visitor<'ast> for SemanticIndexBuilder<'_, 'ast> {
                 self.visit_expr(&node.annotation);
                 if let Some(value) = &node.value {
                     self.visit_expr(value);
+                    if self.is_method_of_class().is_some() {
+                        // Record the right-hand side of the assignment as a standalone expression
+                        // if we're inside a method. This allows type inference to infer the type
+                        // of the value for annotated assignments like `self.CONSTANT: Final = 1`,
+                        // where the type itself is not part of the annotation.
+                        self.add_standalone_expression(value);
+                    }
                 }
 
                 if let ast::Expr::Name(name) = &*node.target {