Support expression defaults in LEAD and LAG - #74

Which issue does this PR close?

Closes no issue.

Rationale for this change

LEAD and LAG currently reject a non-literal third argument even though SQL evaluates the default expression for the current row. This blocks Snowflake-compatible queries such as LAG(value, 1, id * 10).

What changes are included in this PR?

• evaluate non-literal defaults as a per-row window input
• preserve the existing scalar fast path for literal defaults
• support both RESPECT NULLS and IGNORE NULLS
• cast dynamic defaults to the result type
• add focused unit and SQL logic tests

Are there any user-facing changes?

Yes. LEAD and LAG now accept expression defaults and evaluate them against the current row when the requested offset is outside the partition.

How was this tested?

• cargo +1.95.0 test -p datafusion-functions-window
• cargo +1.95.0 test -p datafusion-sqllogictest --test sqllogictests -- lead_lag_expression_default
• cargo +1.95.0 clippy -p datafusion-functions-window --all-targets --all-features -- -D warnings
• cargo +1.95.0 fmt --all -- --check