fix(index): initialize indices on nested fields

[lance-gatekeeper]

Summary

• preserve the canonical schema field path while initializing an index
• use the full path for target validation and scalar or vector index creation
• cover simple nested fields and nested fields that require path quoting

Root cause

initialize_index resolved each source field by ID but then used only the leaf field name. This discarded the parent path and canonical quoting before the target lookup and index initializer ran.

Validation

• cargo test -p lance --lib test_initialize_
• cargo fmt --all -- --check
• cargo clippy --all --tests --benches -- -D warnings

Fixes #8149

[lance-gatekeeper]

✅ Gate recommendation: approve.

Resolving source field IDs to canonical schema paths at the shared initialization boundary preserves parent segments and quoting for target validation and both scalar and vector initialization, without changing stored index metadata or public semantics.

[Xuanwo]

Nice fix!