feat(scan): support validated FileScanTask byte ranges

[kamcheungting-db]

Summary

Allow FileScanTask to represent a validated half-open byte range instead of always representing an entire data file.

• add FileScanTask::MakeSplit, start, length, and is_split
• validate negative, empty, out-of-file, and overflowing ranges
• preserve ranges when parsing and writing REST scan-task JSON
• include ranges in REST task equality
• report each split byte size and estimate row counts additively without overflow
• reject partial tasks in FileScanTaskReader until range-aware file reading is implemented

Behavior

A full-file range is canonicalized to the existing whole-file task representation. An empty range is valid only for an empty whole file. REST responses may omit both start and length for compatibility, but providing only one endpoint, null endpoints, or an invalid range returns a parse error.

For example, two tasks can cover a 10-byte file without overlap:

{"start": 0, "length": 4}
{"start": 4, "length": 6}

Scope

This PR models, validates, and transports byte ranges. It does not enable partial file reads yet. Returning NotSupported prevents a split task from silently rereading the entire file; range-aware format readers will follow separately.

Testing

• ctest --test-dir build-clang -R "^(scan_test|data_test|rest_catalog_test)$" --output-on-failure
• clang-format on the changed C++ sources
• git diff --check origin/main...HEAD