feat(java): add allowExternalBlobOutsideBases to WriteParams

[beinan]

Summary

Add support for writing blob v2 columns with external URI references that are outside registered base paths. This enables use cases like INSERT INTO SELECT across Lance tables where the target table stores external blob references pointing to the source table's blob files instead of copying the actual blob bytes.

Changes

• WriteParams.java: Add allowExternalBlobOutsideBases Optional field, getter, and builder method
• Fragment.java: Pass the new field through createWithFfiArray and createWithFfiStream native methods
• fragment.rs (JNI): Thread the new Optional<Boolean> parameter through all fragment creation functions to extract_write_params
• utils.rs (JNI): Parse the new parameter and set allow_external_blob_outside_bases on Rust WriteParams
• blocking_dataset.rs (JNI): Pass JObject::null() for the new param in Dataset.write() path (not needed there)

Context

This is a prerequisite for lance-spark blob JOIN support (lance-format/lance-spark#355). When blob data flows through Spark's shuffle during JOIN + INSERT INTO, the target table needs to write external blob references pointing to the source table's physical blob files. The Rust BlobPreprocessor already supports this via allow_external_blob_outside_bases, but the Java SDK had no way to set it.

Ref: #6321, #6322

Test plan

• Rust JNI code compiles cleanly (no errors in changed files)
• Java unit tests (CI)

🤖 Generated with Claude Code

[jackye1995]

looks good, pending CI fixes