[swaminimore]

Summary

Fixes #5191

When a union containing a literal of a single-member enum is normalized to the enum class, Pyrefly can incorrectly reject the resulting type against a generic type alias containing that enum literal.

This change updates subset checking to accept the enum class as compatible with its literal member when both refer to the same enum and that enum has exactly one member. Multi-member enums remain subject to the existing compatibility checks.

Regression tests cover the generic type-alias case, direct function arguments, inline enum-literal unions, and compatibility boundaries for multi-member enums.

• I am a human, and I fully understand the code I am submitting.

Test Plan

• cargo test -p pyrefly - passed (9,109 tests passed, 3 ignored).
• cargo fmt --check - passed, with warnings for nightly-only formatting options.
• git diff --check - passed.
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema - passed.
• cargo clippy -p pyrefly --all-targets - completed with one warning in the existing, unrelated pyrefly/benches/micro.rs file.

AI assistance was used during investigation, implementation, and testing. The changes were reviewed and tested locally.

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