fix(cast): correct scientific notation string to decimal parsing (#63085)

[leonard9893]

Problem

CAST('1.4E+4' AS DECIMAL(38, 15)) returns 14.000... instead of 14000.000... in Doris 4.0.5 (regression from 2.1).

Root Cause

be/src/util/string_parser.cpp — string_to_decimal function at line 191 uses a loop variable i that was contaminated by the exponent parsing loop, instead of the preserved end_digit_index.

Introduced by commit c108263.

Fix

Replace i with end_digit_index on line 191.

Fixes #63085

[hello-stephen]

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[leonard9893]

Hi @apache/doris-committers, this PR fixes a regression in DECIMAL parsing of scientific notation introduced by #60004.

CAST('1.4E+4' AS DECIMAL(38,15)) currently returns 14.000... instead of 14000.000... because the loop variable i was contaminated by the exponent parsing loop.

The fix is a one-line change replacing reused i with the preserved end_digit_index in be/src/util/string_parser.cpp. Could someone please take a look? Thanks!