fix: Resolve UnboundLocalError for first_valid in oneOf validator

[Yuki9814]

Description

Fixes a potential UnboundLocalError in the oneOf validator function.

The Bug

In jsonschema/_keywords.py, the oneOf() function uses a for-else loop to find the first valid schema:

for index, subschema in subschemas:
    errs = list(validator.descend(instance, subschema, schema_path=index))
    if not errs:
        first_valid = subschema
        break
    all_errors.extend(errs)
else:
    yield ValidationError(...)  # No valid schema found

more_valid = [...]
if more_valid:
    more_valid.append(first_valid)  # BUG: first_valid may be unbound!

If the for loop completes without finding a valid schema (the else branch executes), first_valid is never assigned. However, the subsequent if more_valid: block unconditionally tries to append first_valid to the list, which would raise UnboundLocalError: local variable 'first_valid' referenced before assignment.

The Fix

Initialize first_valid = None before the loop and check first_valid is not None before using it:

first_valid = None
for index, subschema in subschemas:
    ...

if more_valid and first_valid is not None:
    more_valid.append(first_valid)
    ...

Testing

All 7,288 existing tests pass with this change.