[ty] Add failing mdtest for known `Protocol` panic (#21594)

## Summary

This PR adds a failing mdtest for the panic in
https://github.com/astral-sh/ty/issues/1587. The added snippet currently
panics with this query stacktrace:

```
error[panic]: Panicked at /Users/alexw/.cargo/git/checkouts/salsa-e6f3bb7c2a062968/17bc55d/src/function/execute.rs:321:21 when checking `/Users/alexw/dev/ruff/foo.py`: `ClassLiteral < 'db >::explicit_bases_(Id(4c09)): execute: too many cycle iterations`
info: This indicates a bug in ty.
info: If you could open an issue at https://github.com/astral-sh/ty/issues/new?title=%5Bpanic%5D, we'd be very appreciative!
info: Platform: macos aarch64
info: Version: ruff/0.14.5+105 (d24c891a4 2025-11-22)
info: Args: ["target/debug/ty", "check", "foo.py", "--python-version=3.14"]
info: run with `RUST_BACKTRACE=1` environment variable to show the full backtrace information
info: query stacktrace:
   0: cached_protocol_interface(Id(6805))
             at crates/ty_python_semantic/src/types/protocol_class.rs:795
   1: is_equivalent_to_object_inner(Id(8003))
             at crates/ty_python_semantic/src/types/instance.rs:667
   2: infer_deferred_types(Id(1406))
             at crates/ty_python_semantic/src/types/infer.rs:140
             cycle heads: infer_definition_types(Id(140b)) -> iteration = 200, TypeVarInstance < 'db >::lazy_bound_(Id(5802)) -> iteration = 200
   3: TypeVarInstance < 'db >::lazy_bound_(Id(5803))
             at crates/ty_python_semantic/src/types.rs:8827
   4: infer_definition_types(Id(140c))
             at crates/ty_python_semantic/src/types/infer.rs:94
   5: infer_deferred_types(Id(1405))
             at crates/ty_python_semantic/src/types/infer.rs:140
   6: TypeVarInstance < 'db >::lazy_bound_(Id(5802))
             at crates/ty_python_semantic/src/types.rs:8827
   7: infer_definition_types(Id(140b))
             at crates/ty_python_semantic/src/types/infer.rs:94
   8: infer_scope_types(Id(1000))
             at crates/ty_python_semantic/src/types/infer.rs:70
   9: check_file_impl(Id(c00))
             at crates/ty_project/src/lib.rs:535
```

It's not totally clear to me how to fix this or to what extent it might
be a bug in our `Protocol` internals rather than a bug in our `TypeVar`
internals. (It's sort of interesting that we're trying to evaluate the
upper bound of any `TypeVar`s here!) @carljm suggested that it would be
a good idea to add a failing mdtest in the meantime to document the
panic, which I agree with.

## Test Plan

I verified that we panic on this snippet, and that the test fails if I
remove the `expect-panic` assertion or if I change the asserted error
message.

I experimented with ways of minimizing the snippet further, but I think
any further minimization takes the snippet further away from something a
user would actually be likely to write -- so I think is probably
counterproductive. The failing test added in this PR isn't unreasonable
code at the end of the day; I've seen Python like it in the wild.

crates/ty_python_semantic/resources/mdtest/protocols.md

@@ -3177,6 +3177,33 @@ from ty_extensions import reveal_protocol_interface
 reveal_protocol_interface(Foo)
 ```
 
+## Known panics
+
+### Protocols generic over TypeVars bound to forward references
+
+This test currently panics because the `ClassLiteral::explicit_bases` query fails to converge. See
+issue <https://github.com/astral-sh/ty/issues/1587>.
+
+<!-- expect-panic: execute: too many cycle iterations -->
+
+```py
+from typing import Any, Protocol, TypeVar
+
+T1 = TypeVar("T1", bound="A2[Any]")
+T2 = TypeVar("T2", bound="A1[Any]")
+T3 = TypeVar("T3", bound="B2[Any]")
+T4 = TypeVar("T4", bound="B1[Any]")
+
+class A1(Protocol[T1]):
+    def get_x(self): ...
+
+class A2(Protocol[T2]):
+    def get_y(self): ...
+
+class B1(A1[T3], Protocol[T3]): ...
+class B2(A2[T4], Protocol[T4]): ...
+```
+
 ## TODO
 
 Add tests for: