Fix negative from index handling in INSTR

[lkxdsb]

What changes were proposed in this pull request?

This PR fixes INSTR when the from argument is negative.

• interpret a negative from as a position counted backward from the end of the string
• search backward from that position and return the nth matching occurrence
• keep return values 1-based and return 0 when no occurrence can be found
• apply identical semantics to the String and BinaryString overloads
• preserve the existing behavior for positive from, from = 0, invalid nth, and null arguments
• add both focused unit tests and a local GeaFlow runtime integration test

Why are the changes needed?

The previous implementation always converted from to a zero-based forward-search index:

int fromIndex = from.intValue() - 1;

A negative value therefore entered the invalid-index branch and returned null, instead of locating a starting position from the end of the string and searching backward. This produced incorrect results for the negative-index behavior described in #87.

Fixes #87

Behavior after this change

Expression	Result	Explanation
INSTR('abcabc', 'a', -1, 1)	4	last a when searching backward from the end
INSTR('abcabc', 'a', -1, 2)	1	second a encountered while searching backward
INSTR('abcabc', 'c', -3, 1)	3	search starts at the position selected by -3
INSTR('aaaa', 'aa', -1, 2)	2	overlapping occurrences remain searchable
INSTR('abcabc', 'a', -7, 1)	0	starting position is outside the string

Implementation details

• The String overload uses String.lastIndexOf for the negative search path.
• BinaryString does not provide lastIndexOf, so the implementation retains the last forward match not beyond the requested starting position.
• Advancing the BinaryString scan by one character preserves overlapping matches.
• Positive-index handling remains on the existing forward-search path to minimize behavior changes outside this issue.

How was this PR tested?

Unit test

JAVA_HOME=$(/usr/libexec/java_home -v 11) \
mvn -pl geaflow/geaflow-dsl/geaflow-dsl-plan \
-Dtest=UDFInstrTest test

Result:

Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

The unit test covers negative positions, multiple backward matches, overlapping matches, out-of-range positions, multibyte strings, and both input types.

Local runtime integration test

JAVA_HOME=$(/usr/libexec/java_home -v 11) \
mvn -pl geaflow/geaflow-dsl/geaflow-dsl-runtime \
-Dtest=InstrRuntimeTest test

Result:

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

This test starts the local GeaFlow environment, submits the SQL through GQLPipeLine, executes it with the file connector, and compares the generated Sink output with the expected result:

4,1,3,2,0

Additional checks

• Checkstyle: 0 violations
• Apache RAT: 0 unapproved licenses
• git diff --check: passed