fix: preserve types in NVL and IFNULL

[mikamikasuki]

Which issue does this PR close?

• Closes nvl / ifnull convert decimals to Float64 and timestamps and dates to strings #25947

Rationale for this change

NVL and IFNULL could coerce DECIMAL values to Float64, losing precision, and DATE/TIMESTAMP values to Utf8. This differs from COALESCE and prevents type-specific operations such as timestamp arithmetic.

What changes are included in this PR?

• Use COALESCE's common-type resolution for NVL and IFNULL.
• Preserve the existing Boolean result for all-NULL calls and Boolean-to-numeric fallback behavior.
• Add SQL logic tests for decimal types and precision, date/timestamp types, and timestamp arithmetic.

What is the testing strategy for this PR?

• NVL SQL logic tests, including the existing IFNULL numeric-fallback case.
• Full extended workspace tests passed.
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings

Are there any user-facing changes?

NVL and IFNULL preserve decimal, date, and timestamp types instead of converting them to Float64 or Utf8.

[mikamikasuki]

Closing this as a duplicate of #25964, which addresses the same NVL/IFNULL type coercion issue and includes coverage for decimal precision, temporal types, and existing coercion behavior.