[bfrancojr]

Fixes #166.

You decided #166 as option 2: DatasetReader::Scan keeps honouring the caller's options, and protowire-go changes to match (trendvidia/protowire-go#280, PR trendvidia/protowire-go#281). So cpp's behaviour stays. This PR adds the tests and the header line from the done-when, plus one text fix that came out of measuring the two ports side by side.

The text fix

A validator's refusal or a schema violation was reported with the row's position in front: 2:1: check: id: denied (...), 2:1: PXF schema violations: .... Go returns both as they are, because its atRow moves only a decode error to the row. cpp's AtRow moved every error. Now:

• BindCheckedRow runs the schema check before the decode and the validator after it, and returns their errors unchanged. opts.report still gets the report.
• A hook's error from on_secret_field is a decode error. It keeps the row's position and the column, as in Go.
• Scan checks the column paths before the schema, as Go does. A row type that breaks both gets the column's error.

Evidence

• New test/pxf_dataset_scan_options_test.cc has 7 tests: one per option, plus the hook error and the check order. Each binds one row through Scan and through Parse + BindRow, and requires the two to agree. The protos, rows and exact texts are the ones protowire-go's dataset_scan_options_test.go uses on #281, and those Go tests pass there.
• Without the source change, Validator and SkipValidate fail with the 2:1: prefix. The 4 tests that check an option is honoured pass on the old code too, as expected: cpp already honoured them.
• Locally, ctest passes 742 of 742, and all 7 new tests appear by name. clang-format 18 has been applied, and local clang-tidy 23 reports nothing on dataset_reader.cc and its header.
• The probe from DatasetReader::Scan honours options protowire-go's Scan drops (skip_validate, skip_post_decode, discard_unknown, on_secret_field, validator) #166, re-run against Go #281, gives cpp's verdict in all 12 cases.
• CI on 45a42a2 (head): all 11 checks pass, including CodeQL, ASan + UBSan, clang-tidy 18, the differential against protowire-go, and Windows.

Order

This PR doesn't depend on Go's: cpp's behaviour was already the decided one. Merging #281 is what makes the two ports agree.