fix: always use v2 encoder while blob v2 enabled

[Xuanwo]

This PR fixes a bug where users could encode blob v1 even when blob v2 was enabled. However, our decoder only reads the dataset's configuration lance.blob.version, which can lead to decoding issues.

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Parts of this PR were drafted with assistance from Codex (with gpt-5.2) and fully reviewed and edited by me. I take full responsibility for all changes.

[github-actions]

PR Review

Summary

This PR ensures that when blob v2 is enabled (file version >= 2.2), all blob-marked fields use the v2 encoder regardless of their input type (Binary/LargeBinary or Struct). The key addition is a normalize_blob_v2_input function that converts various input formats to the normalized v2 struct format.

Critical Issues (P0)

1. Potential panic from unchecked cast operations
In normalize_blob_v2_input, the code calls .as_binary::<i64>() and .as_string::<i32>() without checking if the actual array type matches. If the struct has a data field that is not LargeBinary or a uri field that is not Utf8, these will panic:

// blob.rs:455-457
let data_col = data_col.as_binary::<i64>();
let uri_col = uri_col.as_string::<i32>();

Consider using as_binary_opt and as_string_opt with proper error handling, similar to the pattern used in the Binary/LargeBinary branch.

Moderate Issues (P1)

2. Missing unit tests for normalize_blob_v2_input
The new normalize_blob_v2_input function handles several conversion paths (6-field struct passthrough, 2-field struct conversion, Binary, LargeBinary) but has no dedicated unit tests. Only the integration test in take.rs exercises one path. Add unit tests to cover:

• Invalid struct with wrong number of fields
• Struct with missing required fields
• Binary vs LargeBinary input normalization
• Null handling in various paths

3. Test assertion may be fragile
The test test_take_blob_v2_from_legacy_large_binary_on_v2_2 asserts exact field names and positions:

assert_eq!(struct_arr.fields()[0].name(), "kind");
assert_eq!(struct_arr.fields()[1].name(), "position");

Consider using column_by_name for field access instead of positional indexing, matching the pattern used in the encoder itself.