Added shared validator helper that records context for mismatches and eliminates false-positive diffs

[kumarUjjawal]

Which issue does this PR close?

• Closes Improve error message in sqllogictest with <slt:ignore> marker #18878

Rationale for this change

Centralize <slt:ignore> handling so the validator and error output use the same logic, eliminating false-positive diffs.

What changes are included in this PR?

• df_value_validator now records mismatch context (expected/actual snapshots + reason) via a shared helper.
• The sqllogictest runner consumes that context to print [Reason]/[Expected]/[Actual] instead of the old diff.

Are these changes tested?

• Added targeted tests covering ignore markers and trailing-data handling.

Are there any user-facing changes?

Note for the reviewers

• I have understanding of the general issue and the what I wanted to do, but I am not fully confident in my implementation, I also used LLM for the help while structuring parts of the code when my build was failing repeatedly.