feat: add DecodeFieldError/ReadFileError wrappers for error context

[koriyoshi2041]

Summary

Adds two wrapper error types to provide better context when decoding errors occur, as described in #5602:

• DecodeFieldError (lance-encoding/src/decoder.rs): wraps errors with field name and field id. Applied at field scheduler creation in both structural and legacy decode paths (Struct, List, FixedSizeList, Map).
• ReadFileError (lance-file/src/reader.rs): wraps errors with file path. Applied at FileReader::read_tasks and read_stream_projected_blocking entry points. FileReader now stores the file path.

Both implement std::error::Error with proper source chaining and From<...> for lance_core::Error, producing error chains like:

Error: failed to read file 'data.lance'

Caused by:
    0: failed to decode field 'age' (id=10)
    1: number out of range

Design follows the approach from https://sabrinajewson.org/blog/errors as referenced in the issue.

Test plan

• Unit tests for DecodeFieldError display, source chain, and conversion to lance_core::Error (3 tests)
• Unit tests for ReadFileError display, source chain with nested DecodeFieldError, and conversion to lance_core::Error (3 tests)
• cargo fmt --check passes
• cargo clippy --all-targets -p lance-encoding -p lance-file passes
• All existing tests pass

Closes #5602

[github-actions]

PR Review

Good approach following the error wrapping pattern from the linked blog post. The DecodeFieldError / ReadFileError types with proper source() chaining will meaningfully improve debuggability.

Issues

P1 — Incomplete wrapping in legacy decode path

The structural path wraps child field errors for Struct, List, FixedSizeList, and Map. But in the legacy path, only Struct children (line ~990) get wrapped. The legacy create_list_scheduler (line 634) calls create_legacy_field_scheduler for the list's child without wrapping:

let items_scheduler =
    self.create_legacy_field_scheduler(&list_field.children[0], column_infos, buffers)?;

This should also wrap with DecodeFieldError for consistency. Same for any other legacy compound types (FixedSizeList, etc.) that recurse into children.

P1 — io_source loses original error category

Both From<DecodeFieldError> and From<ReadFileError> convert via Error::io_source(). This means if the original error was e.g. Error::invalid_input(...), the wrapped error becomes an IO variant. This changes the error category, which could break callers that match on error variants for control flow. Consider preserving the original error's category or using a category-agnostic wrapping mechanism.

Nit

The #[cfg(test)] block had a comment // test coalesce indices to ranges removed — this looks like an accidental drive-by deletion (the comment described the test below it).

[koriyoshi2041]

Thanks for the thorough review! All three issues have been addressed in the latest commit:

1. P1 — Legacy decode path: Added DecodeFieldError wrapping in create_list_scheduler for child field errors, matching the structural path's behavior.

2. P1 — Error category preservation: Changed From<DecodeFieldError> and From<ReadFileError> to use Self::wrapped() instead of Self::io_source(), so the original error category is preserved in the chain.

3. Nit — Restored comment: Put back the // test coalesce indices to ranges comment above mod tests.

All 428 tests pass (362 lance-encoding + 66 lance-file).