[ty] Add subdiagnostic suggestion to unresolved-reference diagnostic when variable exists on self (#18444)

## Summary

Closes https://github.com/astral-sh/ty/issues/502.

In the following example:
```py
class Foo:
    x: int

    def method(self):
        y = x
```
The user may intended to use `y = self.x` in `method`. 

This is now added as a subdiagnostic in the following form : 

`info: An attribute with the same name as 'x' is defined, consider using
'self.x'`

## Test Plan

Added mdtest with snapshot diagnostics.

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -928,6 +928,42 @@ def _(flag1: bool, flag2: bool):
     reveal_type(C5.attr1)  # revealed: Unknown | Literal["metaclass value", "class value"]
 ```
 
+## Invalid access to attribute
+
+<!-- snapshot-diagnostics -->
+
+If a non-declared variable is used and an attribute with the same name is defined and accessible,
+then we emit a subdiagnostic suggesting the use of `self.`.
+(`An attribute with the same name as 'x' is defined, consider using 'self.x'` in these cases)
+
+```py
+class Foo:
+    x: int
+
+    def method(self):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+```py
+class Foo:
+    x: int = 1
+
+    def method(self):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
+```py
+class Foo:
+    def __init__(self):
+        self.x = 1
+
+    def method(self):
+        # error: [unresolved-reference] "Name `x` used when not defined"
+        y = x
+```
+
 ## Unions of attributes
 
 If the (meta)class is a union type or if the attribute on the (meta) class has a union type, we

crates/ty_python_semantic/resources/mdtest/snapshots/attributes.md_-_Attributes_-_Invalid_access_to_at…_(5457445ffed43a87).snap

@@ -0,0 +1,82 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: attributes.md - Attributes - Invalid access to attribute
+mdtest path: crates/ty_python_semantic/resources/mdtest/attributes.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | class Foo:
+ 2 |     x: int
+ 3 | 
+ 4 |     def method(self):
+ 5 |         # error: [unresolved-reference] "Name `x` used when not defined"
+ 6 |         y = x
+ 7 | class Foo:
+ 8 |     x: int = 1
+ 9 | 
+10 |     def method(self):
+11 |         # error: [unresolved-reference] "Name `x` used when not defined"
+12 |         y = x
+13 | class Foo:
+14 |     def __init__(self):
+15 |         self.x = 1
+16 | 
+17 |     def method(self):
+18 |         # error: [unresolved-reference] "Name `x` used when not defined"
+19 |         y = x
+```
+
+# Diagnostics
+
+```
+error[unresolved-reference]: Name `x` used when not defined
+ --> src/mdtest_snippet.py:6:13
+  |
+4 |     def method(self):
+5 |         # error: [unresolved-reference] "Name `x` used when not defined"
+6 |         y = x
+  |             ^
+7 | class Foo:
+8 |     x: int = 1
+  |
+info: An attribute `x` is available, consider using `self.x`
+info: rule `unresolved-reference` is enabled by default
+
+```
+
+```
+error[unresolved-reference]: Name `x` used when not defined
+  --> src/mdtest_snippet.py:12:13
+   |
+10 |     def method(self):
+11 |         # error: [unresolved-reference] "Name `x` used when not defined"
+12 |         y = x
+   |             ^
+13 | class Foo:
+14 |     def __init__(self):
+   |
+info: An attribute `x` is available, consider using `self.x`
+info: rule `unresolved-reference` is enabled by default
+
+```
+
+```
+error[unresolved-reference]: Name `x` used when not defined
+  --> src/mdtest_snippet.py:19:13
+   |
+17 |     def method(self):
+18 |         # error: [unresolved-reference] "Name `x` used when not defined"
+19 |         y = x
+   |             ^
+   |
+info: An attribute `x` is available, consider using `self.x`
+info: rule `unresolved-reference` is enabled by default
+
+```