Add interpreter regression tests for joined-column option typing

[cmontella]

Motivation

• Cover a regression where joined-table column lookups should yield option-typed values (u8?) and unwrap to u8 when matched.
• Capture a coercion bug where an option-match wildcard arm (| * => 0.) is inferred as f64 instead of being coerced to the first arm's integer kind (u8).

Description

• Added three tests to tests/interpreter.rs (guarded by #[cfg(all(feature = "u8", feature = "u64"))]) that exercise joined-column lookups and option matching.
• The new tests are interpret_joined_column_lookup_infers_option_for_present_value, interpret_joined_column_lookup_infers_option_for_missing_value, and interpret_joined_column_option_match_uses_first_arm_type_for_wildcard_coercion, and they assert expected u8 results via the test_interpreter! macro.
• The tests exercise both present and missing joined values and a failing coercion case where the wildcard fallback should be coerced to u8.

Testing

• Ran cargo test --test interpreter interpret_joined_column -- --nocapture, which executed the three new tests.
• Two tests passed (interpret_joined_column_lookup_infers_option_for_present_value, interpret_joined_column_lookup_infers_option_for_missing_value) and one test (interpret_joined_column_option_match_uses_first_arm_type_for_wildcard_coercion) failed, reproducing the reported coercion bug.
• The failure is expected and documents the bug for follow-up fixes so the test suite can be used to verify a future fix.

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Codex Task