fix(java): carry alterColumns cast type across FFI via C Data Interface

[puchengy]

Problem

Dataset.alterColumns(...) with a castTo(...) alteration silently drops the cast: the commit lands (version bumps) but the stored column type is unchanged.

Root cause is in the JNI create_column_alteration (java/lance-jni/src/blocking_dataset.rs). The cast target type was marshalled by calling the Java ArrowType.toString() (e.g. "Int(64, true)", "FloatingPoint(DOUBLE)") and parsing the string with arrow_schema::DataType::from_str, then discarding any parse error with .ok():

let data_type_str: String = env.get_string(&jstring)?.into();
DataType::from_str(&data_type_str)
    .map_err(|e| Error::input_error(e.to_string()))
    .ok()   // parse failure -> None -> cast silently dropped

DataType's FromStr grammar does not accept the Debug-style strings that ArrowType.toString() produces for parameterized types, so data_type became None for anything beyond the few types whose toString() happens to match (e.g. Utf8). The existing DatasetTest.testAlterColumns only asserted field names after a cast, so the dropped type went unnoticed.

Fix

Transfer the cast target type through the Arrow C Data Interface, mirroring the existing addColumns(Schema) path:

• Java (Dataset.alterColumns): export one field per requested cast — in the same order as the alterations — into an ArrowSchema, and pass its memory address to the native method.
• JNI (inner_alter_columns): import the schema via FFI_ArrowSchema and attach each imported DataType to the corresponding ColumnAlteration.

Rename-only and nullability-only alterations are unaffected. Removes the now-unused DataType / FromStr imports.

Test

Adds DatasetTest.testAlterColumnsCastType: widens id from Int32 to Int64, then does a combined rename+cast, asserting the resulting Arrow type (not just the field name).

Context

Surfaced while implementing schema-evolution DDLs in lance-spark (lance-format/lance-spark#752), where ALTER COLUMN ... TYPE had to be rejected because of this bug. With this fix released, lance-spark can enable type changes.

🤖 Generated with Claude Code

[lance-gatekeeper]

✅ Gate recommendation: approve.

The typed Arrow C Data transfer fixes the lossy Java-to-Rust boundary without extending display-string parsing, while preserving rename-only and nullability-only alterations. The regression coverage exercises a parameterized cast and a combined rename/cast.

[westonpace]

We do have String<->DataType somewhere I think but C FFI seems reasonable to me as well. No change to public APIs that I can see.