[ty] Avoid duplicate diagnostics during multi-inference of standalone expressions (#21056)

## Summary

Resolves https://github.com/astral-sh/ty/issues/1428.

crates/ty_python_semantic/resources/mdtest/call/union.md

@@ -284,6 +284,8 @@ def _(flag: bool):
 
 Diagnostics unrelated to the type-context are only reported once:
 
+`expression.py`:
+
 ```py
 def f[T](x: T) -> list[T]:
     return [x]
@@ -307,3 +309,18 @@ def _(x: int):
     # error: [possibly-unresolved-reference] "Name `z` used when possibly not defined"
     y(f(True), [z])
 ```
+
+`standalone_expression.py`:
+
+```py
+def f(_: str): ...
+def g(_: str): ...
+def _(a: object, b: object, flag: bool):
+    if flag:
+        x = f
+    else:
+        x = g
+
+    # error: [unsupported-operator] "Operator `>` is not supported for types `object` and `object`"
+    x(f"{'a' if a > b else 'b'}")
+```

crates/ty_python_semantic/src/types/context.rs

@@ -97,8 +97,10 @@ impl<'db, 'ast> InferContext<'db, 'ast> {
     }
 
     pub(crate) fn extend(&mut self, other: &TypeCheckDiagnostics) {
+        if !self.is_in_multi_inference() {
             self.diagnostics.get_mut().extend(other);
         }
+    }
 
     pub(super) fn is_lint_enabled(&self, lint: &'static LintMetadata) -> bool {
         LintDiagnosticGuardBuilder::severity_and_source(self, lint).is_some()