fix: guard array_position start_from underflow

[Sean-Kenneth-Doherty]

Which issue does this PR close?

• Closes panic: array_position underflows start_from at i64::MIN #22220.

Rationale for this change

array_position converts the optional 1-indexed start_from argument to a 0-indexed offset by subtracting one. When start_from is i64::MIN, that subtraction can underflow and panic before DataFusion can return a normal error.

What changes are included in this PR?

• Adds a shared checked conversion for array_position start_from values.
• Uses it in both scalar and array start_from paths.
• Adds regression coverage for the i64::MIN boundary.

Are these changes tested?

• cargo fmt --all
• TMPDIR=/home/sean/Projects/datafusion-array-position-min/target/tmp cargo test -p datafusion-functions-nested position::tests::test_array_position_start_from_min_value -- --nocapture
• TMPDIR=/home/sean/Projects/datafusion-array-position-min/target/tmp cargo test --profile=ci --test sqllogictests -- array/array_position.slt
• TMPDIR=/home/sean/Projects/datafusion-array-position-min/target/tmp cargo clippy --all-targets --all-features -- -D warnings
• git diff --check

I also manually checked the issue reproducer no longer panics: EXPLAIN SELECT array_position([1], 1, -9223372036854775808); now returns a plan, while plain execution returns Execution error: start_from out of bounds: -9223372036854775808.

Are there any user-facing changes?

Invalid array_position start_from values at the i64::MIN boundary now return a normal DataFusion error instead of panicking.

[Sean-Kenneth-Doherty]

Fresh validation on the PR head (862c36ee8):

• TMPDIR=/home/sean/Projects/datafusion-array-position-min/target/tmp cargo test -p datafusion-functions-nested position::tests::test_array_position_start_from_min_value -- --nocapture -> passed (1 passed)
• TMPDIR=/home/sean/Projects/datafusion-array-position-min/target/tmp cargo test --profile=ci --test sqllogictests -- array/array_position.slt -> passed (1/1 files completed)
• cargo fmt --all -- --check -> passed
• git diff --check origin/main...HEAD -> clean

GitHub Actions only ran the labeler for this PR, so this keeps the regression evidence visible while the branch waits for review.