fix: preserve Spark next_day whitespace validation

[xfocus3]

Which issue does this PR close?

• Closes [datafusion-spark] next_day trims whitespace from dayOfWeek; Spark does not #22717.

Rationale for this change

Spark does not trim dayOfWeek before matching it in next_day, but datafusion-spark currently does. That makes values like ' MO ' succeed in DataFusion even though Spark treats them as invalid.

What changes are included in this PR?

• remove the .trim() call from spark_next_day
• add a regression test proving whitespace-padded day names are rejected

Are these changes tested?

• cargo test -p datafusion-spark next_day_rejects_whitespace_padded_day_names -- --nocapture
• cargo test -p datafusion-spark
• cargo fmt --all --check
• cargo clippy -p datafusion-spark --all-targets --all-features --no-deps -- -D warnings

Note: the broader package clippy invocation still reports an existing unused import warning in untouched datafusion/core/src/execution/session_state.rs on current main.

Are there any user-facing changes?

Behavior now matches Spark for whitespace-padded dayOfWeek inputs in next_day.