Object of class `FunctionType` has no attribute `return_value` [missing-attribute]

### Describe the Bug

in pytest env, we can set .return_value to change the return value.

https://github.com/langgenius/dify/pull/39876/changes#diff-11f51968a981aae553847d0824bf7e3f0750f5c1145432e57ec7c55eb727b216L157

But this causes some false positive.

### Sandbox Link

_No response_

### (Only applicable for extension issues) IDE Information

_No response_

Start with the linked Dify pull-request diff, especially the referenced line, and reproduce the `FunctionType.return_value` diagnostic in a pytest environment. Trace why Pyrefly treats this assignment as a missing attribute and verify that the false positive is no longer reported without weakening valid diagnostics.

Written by the indexing model from the issue text.