fix: error on negative ln inputs

[puneetdixit200]

Which issue does this PR close?

• Closes PostgreSQL compatibility: ln(-1.0::float8) should error, not return NaN #22271.

Rationale for this change

ln currently delegates directly to Rust floating-point ln, so negative inputs produce NaN. PostgreSQL-style semantics for this case should return a domain error instead.

What changes are included in this PR?

• Moves ln from the shared unary math macro to a focused implementation.
• Returns a compute error for negative Float32/Float64 inputs while preserving the existing ln(0) -> -Infinity behavior.
• Adds SQL and unit coverage for the negative-input behavior.

Are these changes tested?

Yes:

• cargo test --profile=ci --test sqllogictests -- scalar.slt
• cargo test --profile=ci -p datafusion-functions math::ln
• cargo fmt --all
• git diff --check
• cargo clippy --profile=ci --all-targets --all-features -- -D warnings

Are there any user-facing changes?

Yes. ln now returns an error for negative floating-point inputs instead of NaN.

AI assistance: Used OpenAI GPT-5 to help draft and check this focused change; I reviewed the code and tests.

[kumarUjjawal]

There's already a pr for this issue #22276