[ty] Allow unions including Any/Unknown as bases (#18094)

## Summary

Alternative fix for https://github.com/astral-sh/ty/issues/312

## Test Plan

New Markdown test

crates/ty_python_semantic/resources/mdtest/mro.md

@@ -256,6 +256,25 @@ class Foo(x): ...
 reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]
 ```
 
+## `__bases__` is a union of a dynamic type and valid bases
+
+If a dynamic type such as `Any` or `Unknown` is one of the elements in the union, and all other
+types *would be* valid class bases, we do not emit an `invalid-base` diagnostic and use the dynamic
+type as a base to prevent further downstream errors.
+
+```py
+from typing import Any
+
+def _(flag: bool, any: Any):
+    if flag:
+        Base = any
+    else:
+        class Base: ...
+
+    class Foo(Base): ...
+    reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Any, <class 'object'>]
+```
+
 ## `__bases__` includes multiple `Union`s
 
 ```py

crates/ty_python_semantic/resources/mdtest/snapshots/mro.md_-_Method_Resolution_Or…_-_`__bases__`_lists_wi…_(ea7ebc83ec359b54).snap

@@ -140,43 +140,6 @@ info[revealed-type]: Revealed type
 
 ```
 
-```
-error[duplicate-base]: Duplicate base class `Eggs`
-  --> src/mdtest_snippet.py:16:7
-   |
-14 |   # error: [duplicate-base] "Duplicate base class `Spam`"
-15 |   # error: [duplicate-base] "Duplicate base class `Eggs`"
-16 |   class Ham(
-   |  _______^
-17 | |     Spam,
-18 | |     Eggs,
-19 | |     Bar,
-20 | |     Baz,
-21 | |     Spam,
-22 | |     Eggs,
-23 | | ): ...
-   | |_^
-24 |
-25 |   # fmt: on
-   |
-info: The definition of class `Ham` will raise `TypeError` at runtime
-  --> src/mdtest_snippet.py:18:5
-   |
-16 | class Ham(
-17 |     Spam,
-18 |     Eggs,
-   |     ---- Class `Eggs` first included in bases list here
-19 |     Bar,
-20 |     Baz,
-21 |     Spam,
-22 |     Eggs,
-   |     ^^^^ Class `Eggs` later repeated here
-23 | ): ...
-   |
-info: rule `duplicate-base` is enabled by default
-
-```
-
 ```
 error[duplicate-base]: Duplicate base class `Spam`
   --> src/mdtest_snippet.py:16:7
@@ -215,6 +178,43 @@ info: rule `duplicate-base` is enabled by default
 
 ```
 
+```
+error[duplicate-base]: Duplicate base class `Eggs`
+  --> src/mdtest_snippet.py:16:7
+   |
+14 |   # error: [duplicate-base] "Duplicate base class `Spam`"
+15 |   # error: [duplicate-base] "Duplicate base class `Eggs`"
+16 |   class Ham(
+   |  _______^
+17 | |     Spam,
+18 | |     Eggs,
+19 | |     Bar,
+20 | |     Baz,
+21 | |     Spam,
+22 | |     Eggs,
+23 | | ): ...
+   | |_^
+24 |
+25 |   # fmt: on
+   |
+info: The definition of class `Ham` will raise `TypeError` at runtime
+  --> src/mdtest_snippet.py:18:5
+   |
+16 | class Ham(
+17 |     Spam,
+18 |     Eggs,
+   |     ---- Class `Eggs` first included in bases list here
+19 |     Bar,
+20 |     Baz,
+21 |     Spam,
+22 |     Eggs,
+   |     ^^^^ Class `Eggs` later repeated here
+23 | ): ...
+   |
+info: rule `duplicate-base` is enabled by default
+
+```
+
 ```
 info[revealed-type]: Revealed type
   --> src/mdtest_snippet.py:27:13

crates/ty_python_semantic/src/types/class_base.rs

@@ -123,7 +123,32 @@ impl<'db> ClassBase<'db> {
                     Some(valid_element)
                 }
             }
-            Type::Union(_) => None, // TODO -- forces consideration of multiple possible MROs?
+            Type::Union(union) => {
+                // We do not support full unions of MROs (yet). Until we do,
+                // support the cases where one of the types in the union is
+                // a dynamic type such as `Any` or `Unknown`, and all other
+                // types *would be* valid class bases. In this case, we can
+                // "fold" the other potential bases into the dynamic type,
+                // and return `Any`/`Unknown` as the class base to prevent
+                // invalid-base diagnostics and further downstream errors.
+                let Some(Type::Dynamic(dynamic)) = union
+                    .elements(db)
+                    .iter()
+                    .find(|elem| matches!(elem, Type::Dynamic(_)))
+                else {
+                    return None;
+                };
+
+                if union
+                    .elements(db)
+                    .iter()
+                    .all(|elem| ClassBase::try_from_type(db, *elem).is_some())
+                {
+                    Some(ClassBase::Dynamic(*dynamic))
+                } else {
+                    None
+                }
+            }
             Type::NominalInstance(_) => None, // TODO -- handle `__mro_entries__`?
             Type::PropertyInstance(_) => None,
             Type::Never