feat: report empty required and recommended files

[mackenziereading19]

Summary

This PR addresses #1853 by distinguishing GTFS files that are present with valid headers but contain no data rows.

The loader already treats a completely empty/headerless CSV as EmptyFileNotice. However, a headers-only file is considered parsable and previously produced no notice even when the file was required or recommended.

This change adds:

• RequiredFileEmptyNotice (ERROR) for required files with headers but no data rows;
• RecommendedFileEmptyNotice (WARNING) for recommended files with headers but no data rows;
• no new notice for optional files with headers but no data rows;
• updated EmptyFileNotice documentation so it only describes genuinely headerless files.

The parsed table remains PARSABLE_HEADERS_AND_ROWS in all headers-only cases because the file structure and headers are valid; the new notices describe the file-content requirement rather than reclassifying the table as missing or unparsable.

Tests

Added regression coverage for:

• required headers-only file -> RequiredFileEmptyNotice;
• recommended headers-only file -> RecommendedFileEmptyNotice;
• optional headers-only file -> no empty-file notice.

Local verification:

• full :core:test: pass;
• targeted NoticeReferenceTest: pass;
• :core:spotlessCheck and :main:spotlessCheck: pass;
• git diff --check: pass.

Fixes #1853