test: cover regexp_replace multiline flag

[Sean-Kenneth-Doherty]

Which issue does this PR close?

• Closes PostgreSQL compatibility: regexp_replace should honor PostgreSQL multiline flag m #22258.

Rationale for this change

PostgreSQL-compatible regexp_replace should honor the multiline flag m so anchors like ^ can match after newlines. The issue report covers regexp_replace(E'a\nb', '^b', 'x', 'm'), which should produce a\nx.

Current main already returns the PostgreSQL-compatible result, so this PR adds explicit regression coverage to keep that behavior from drifting.

What changes are included in this PR?

• Adds Rust unit coverage for scalar/static regexp_replace with flag m.
• Adds sqllogictest coverage for m and gm multiline flag behavior.

Are these changes tested?

• cargo fmt --all
• TMPDIR=/home/sean/Projects/datafusion-regexp-replace/target/tmp cargo test -p datafusion-functions regex::regexpreplace::tests::test_static_pattern_regexp_replace_multiline_flag -- --nocapture
• TMPDIR=/home/sean/Projects/datafusion-regexp-replace/target/tmp cargo test --profile=ci --test sqllogictests -- regexp/regexp_replace.slt
• TMPDIR=/home/sean/Projects/datafusion-regexp-replace/target/tmp cargo clippy --all-targets --all-features -- -D warnings
• git diff --check

Are there any user-facing changes?

No behavior change intended. This adds regression coverage for PostgreSQL-compatible multiline regexp_replace behavior.

[Sean-Kenneth-Doherty]

Fresh validation on the PR head (b50d56520):

• TMPDIR=/home/sean/Projects/datafusion-regexp-replace/target/tmp cargo test -p datafusion-functions regex::regexpreplace::tests::test_static_pattern_regexp_replace_multiline_flag -- --nocapture -> passed (1 passed)
• TMPDIR=/home/sean/Projects/datafusion-regexp-replace/target/tmp cargo test --profile=ci --test sqllogictests -- regexp/regexp_replace.slt -> passed (1/1 files completed)
• cargo fmt --all -- --check -> passed
• git diff --check origin/main...HEAD -> clean

GitHub Actions only ran the labeler for this PR, so this leaves current regression evidence visible for review.