[ty] Fix panic when attempting to validate the members of a protocol that inherits from a protocol in another module (#20956)

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -1114,6 +1114,8 @@ it's a large section).
 
 <!-- snapshot-diagnostics -->
 
+`a.py`:
+
 ```py
 from typing import Protocol
 
@@ -1140,6 +1142,31 @@ class A(Protocol):
         pass
 ```
 
+Validation of protocols that had cross-module inheritance used to break, so we test that explicitly
+here too:
+
+`b.py`:
+
+```py
+from typing import Protocol
+
+# Ensure the number of scopes in `b.py` is greater than the number of scopes in `c.py`:
+class SomethingUnrelated: ...
+
+class A(Protocol):
+    x: int
+```
+
+`c.py`:
+
+```py
+from b import A
+from typing import Protocol
+
+class C(A, Protocol):
+    x = 42  # fine, due to declaration in the base class
+```
+
 ## Equivalence of protocols
 
 ```toml

crates/ty_python_semantic/resources/mdtest/snapshots/protocols.md_-_Protocols_-_Diagnostics_for_prot…_(585a3e9545d41b64).snap

@@ -9,7 +9,7 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/protocols.md
 
 # Python source files
 
-## mdtest_snippet.py
+## a.py
 
 ```
  1 | from typing import Protocol
@@ -37,11 +37,33 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/protocols.md
 23 |         pass
 ```
 
+## b.py
+
+```
+1 | from typing import Protocol
+2 | 
+3 | # Ensure the number of scopes in `b.py` is greater than the number of scopes in `c.py`:
+4 | class SomethingUnrelated: ...
+5 | 
+6 | class A(Protocol):
+7 |     x: int
+```
+
+## c.py
+
+```
+1 | from b import A
+2 | from typing import Protocol
+3 | 
+4 | class C(A, Protocol):
+5 |     x = 42  # fine, due to declaration in the base class
+```
+
 # Diagnostics
 
 ```
 warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the body of a protocol class
-  --> src/mdtest_snippet.py:12:5
+  --> src/a.py:12:5
    |
 11 |     # error: [ambiguous-protocol-member]
 12 |     a = None  # type: int
@@ -50,7 +72,7 @@ warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the
 14 |     b = ...  # type: str
    |
 info: Assigning to an undeclared variable in a protocol class leads to an ambiguous interface
- --> src/mdtest_snippet.py:6:7
+ --> src/a.py:6:7
   |
 4 |     return True
 5 |
@@ -66,7 +88,7 @@ info: rule `ambiguous-protocol-member` is enabled by default
 
 ```
 warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the body of a protocol class
-  --> src/mdtest_snippet.py:14:5
+  --> src/a.py:14:5
    |
 12 |     a = None  # type: int
 13 |     # error: [ambiguous-protocol-member]
@@ -76,7 +98,7 @@ warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the
 16 |     if coinflip():
    |
 info: Assigning to an undeclared variable in a protocol class leads to an ambiguous interface
- --> src/mdtest_snippet.py:6:7
+ --> src/a.py:6:7
   |
 4 |     return True
 5 |
@@ -92,7 +114,7 @@ info: rule `ambiguous-protocol-member` is enabled by default
 
 ```
 warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the body of a protocol class
-  --> src/mdtest_snippet.py:17:9
+  --> src/a.py:17:9
    |
 16 |     if coinflip():
 17 |         c = 1  # error: [ambiguous-protocol-member]
@@ -101,7 +123,7 @@ warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the
 19 |         c = 2
    |
 info: Assigning to an undeclared variable in a protocol class leads to an ambiguous interface
- --> src/mdtest_snippet.py:6:7
+ --> src/a.py:6:7
   |
 4 |     return True
 5 |
@@ -117,7 +139,7 @@ info: rule `ambiguous-protocol-member` is enabled by default
 
 ```
 warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the body of a protocol class
-  --> src/mdtest_snippet.py:22:9
+  --> src/a.py:22:9
    |
 21 |     # error: [ambiguous-protocol-member]
 22 |     for d in range(42):
@@ -125,7 +147,7 @@ warning[ambiguous-protocol-member]: Cannot assign to undeclared variable in the
 23 |         pass
    |
 info: Assigning to an undeclared variable in a protocol class leads to an ambiguous interface
- --> src/mdtest_snippet.py:6:7
+ --> src/a.py:6:7
   |
 4 |     return True
 5 |