fix: EnumTransformer.to_literal accepts a string matching an enum value

[1fanwang]

Why are the changes needed?

EnumTransformer.assert_type accepts a raw string that matches one of the enum's values (e.g. an enum default supplied as a string in an execution/launch config). EnumTransformer.to_literal rejects that same string with Expected an enum. So a matching-string enum value passes type-checking and then fails at serialization — an inconsistency between the two methods.

What changes were proposed in this pull request?

EnumTransformer.to_literal now accepts a string that matches one of the enum's values, serializing it to the corresponding string literal — consistent with assert_type. A non-matching string is still rejected.

How was this patch tested?

Added test_enum_to_literal_accepts_matching_string.

pytest tests/flytekit/unit/core/test_type_engine.py -k test_enum_to_literal_accepts_matching_string

Before:

>       raise TypeTransformerFailedError("Expected an enum")
E       flytekit.core.type_engine.TypeTransformerFailedError: Expected an enum
FAILED tests/flytekit/unit/core/test_type_engine.py::test_enum_to_literal_accepts_matching_string

After:

1 passed

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• All new and existing tests passed.
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