fix: sync structured primary key fields after metadata update

[wombatu-kun]

Summary
Fixes #6324

UpdateConfig.fieldMetadataUpdates() correctly persists lance-schema:unenforced-primary-key and lance-schema:unenforced-primary-key:position in the field's metadata HashMap, but the corresponding structured field unenforced_primary_key_position (used for protobuf serialization and is_unenforced_primary_key()) was never updated. This caused primary key information set via metadata updates to be lost on the next dataset reload.

Root cause
apply_update_map() in transaction.rs only modifies field.metadata (a HashMap<String, String>) without syncing the structured Field.unenforced_primary_key_position field that drives protobuf serialization and the is_unenforced_primary_key() API.

Fix

• Added Field::sync_embedded_metadata() that re-parses well-known metadata keys (lance-schema:unenforced-primary-key, lance-schema:unenforced-primary-key:position) and updates the corresponding structured fields.
• Call sync_embedded_metadata() after every apply_update_map() on field metadata in Transaction::build_manifest().
• Refactored TryFrom<&ArrowField> for Field to reuse sync_embedded_metadata() instead of duplicating the parsing logic.

[codecov]

Codecov Report

❌ Patch coverage is 99.55157% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-core/src/datatypes/field.rs	93.75%	0 Missing and 1 partial ⚠️

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