feat: not found errors with suggestions

[wjones127]

Closes #5642

[github-actions]

PR Review: feat: not found errors with suggestions

P0 Issues (Critical Bugs)

1. Duplicate code and unreachable statements in rust/lance-index/src/lib.rs

Lines 496-502 in the diff show duplicated code that will never execute:

Err(Error::invalid_input(error_msg, location!()));  // This returns Err but result is ignored
lance_core::levenshtein::find_best_suggestion(value, &valid_index_types);  // Unreachable
let mut error_msg = format!("invalid index type: {}", value);  // Unreachable - redeclares error_msg
if let Some(suggestion) = suggestion {  // Uses wrong suggestion variable
    error_msg = format!("{}. Did you mean '{}'?", error_msg, suggestion);
}
Err(Error::invalid_input(error_msg, location!()))

The first Err() statement returns early but the result is discarded (statement ends with ;). The code after it is dead code. The fix should remove lines 496-501 and keep only the final Err return.

2. Python test assertions are inside with pytest.raises() block (python/python/tests/test_dataset.py)

Lines 55-61 and 64-70 in the test show assertions placed inside the with pytest.raises() block:

with pytest.raises(FieldNotFoundError) as e:
    ds.scanner(columns=["non_existent_field"]).to_table()
    # These assertions never execute because the line above raises
    expected = (...)
    assert str(e.value) == expected

The assertions after to_table() will never run because the exception is raised. Move assertions outside the with block.

3. Test file committed to repo: rust/test_error_suggestions.rs

This appears to be a development test file that shouldn't be committed. It's not part of any crate and won't compile as-is (wrong imports, missing dependencies).

P1 Issues (Should Fix)

4. Levenshtein tests have incorrect assertions (rust/lance-core/src/levenshtein.rs)

Line 425-426:

assert_eq!(levenshtein_distance("vector", "vector"), 0);
assert_eq!(levenshtein_distance("vector", "vector"), 1);  // Should be different inputs

Both lines compare "vector" to "vector" but expect different results (0 and 1). The second line appears to be a copy-paste error - likely should test "vector" vs "vactor" or similar.

5. Unused error type added to lance-namespace (rust/lance-namespace/src/error.rs)

TableColumnNotFoundWithSuggestionError and NamespaceError::TableColumnNotFoundWithSuggestion are added but never used in the codebase. Consider removing until needed, or add the actual usage.

6. Result type alias change may break downstream code (rust/lance-core/src/error.rs)

Line 299: pub type Result<T, E = Error> = std::result::Result<T, E>;

This changes Result<T> to have a default second generic parameter. While backwards compatible in most cases, this could cause issues if any code relies on the exact type signature.