Apply field renames when disambiguating on a Literal field

[onk3sh]

Fixes #779.

create_default_dis_func computes attribute overrides up front, but the use_literals branch never applied them: it built its candidate discriminator names from the raw at.name, and the generated dis_func looked the value up as data[best_discriminator] using that same raw name. A union whose Literal discriminator field was renamed with override(rename=...) therefore failed to structure with a bare KeyError, even though each member structured correctly on its own.

The unique-key branch further down already routes names through _usable_attribute_names, and test_field_renaming covers it. This brings the literal strategy in line.

Before

@define
class A:
    kind: Literal["a"]
    a_val: int

@define
class B:
    kind: Literal["b"]
    b_val: int

c = Converter()
for cl in (A, B):
    c.register_structure_hook(
        cl, make_dict_structure_fn(cl, c, kind=override(rename="type"))
    )

c.structure({"type": "a", "a_val": 1}, A)            # A(kind='a', a_val=1)
c.structure({"type": "a", "a_val": 1}, Union[A, B])  # KeyError: 'kind'

After

Both return A(kind='a', a_val=1).

The change

Candidate names are now built from the overridden names, with a per-class back_map from the payload name to the original attribute name so the Literal args are still read off the right field. The discriminator set is still the intersection across all members, so it only matches when every member agrees on the payload name.

Testing

test_renamed_literal_discriminator added next to the existing test_field_renaming. It fails on main with KeyError: 'kind' across all four converter parametrizations and passes with the fix. Full suite: 994 passed, 15 xfailed, no regressions. ruff format --check and ruff check are clean.

🤖 Generated with Claude Code

[codspeed]

Merging this PR will not alter performance

✅ 64 untouched benchmarks

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_{Comparing onk3sh:fix/literal-disambiguator-rename (07bea8a) with main (5bf7c97)}