Guard to_timestamp decimal overflow

[Sean-Kenneth-Doherty]

Which issue does this PR close?

• Closes panic: to_timestamp overflows converting large Decimal128 to nanoseconds #22213

Rationale for this change

to_timestamp converted Decimal128 inputs to nanoseconds with unchecked i128 multiplication followed by an as i64 cast. Large Decimal128 values could overflow during nanosecond scaling, causing a panic in debug builds or a wrapped timestamp value in release builds.

What changes are included in this PR?

• Convert Decimal128-to-nanoseconds scaling to checked arithmetic.
• Return a DataFusion error when the scaled value cannot fit in timestamp nanoseconds.
• Cover both scalar and array Decimal128 overflow paths.
• Add a sqllogictest regression for the reported query shape.

Are these changes tested?

• cargo fmt --check
• git diff --check
• CARGO_TARGET_DIR=/home/sean/Projects/datafusion-runtime-set-nonascii/target CARGO_BUILD_JOBS=2 cargo test -p datafusion-functions --lib to_timestamp_decimal128
• CARGO_TARGET_DIR=/home/sean/Projects/datafusion-runtime-set-nonascii/target CARGO_BUILD_JOBS=2 cargo clippy -p datafusion-functions --lib -- -D warnings