Fix return value incorrect when fromIndex parameter is negative

For udf `Instr` when the `fromIndex < 0`, current return value is incorrect. It should searches backward from the resulting position.

Start at the UDF `Instr` implementation and locate its existing handling of `fromIndex`; reproduce the incorrect result when the parameter is negative. Check for related tests, then add a regression case that verifies searching backward from the resulting position and confirm the corrected return value.

Written by the indexing model from the issue text.