[ISSUE-789] Add TRIM, LPAD, and RPAD string functions

[914zzzz]

Closes #789.

What changed

• Adds and registers LPAD and RPAD with String and BinaryString overloads.
• Uses Unicode code-point lengths for truncation and padding.
• Keeps the existing Calcite TRIM operator instead of registering a duplicate UDF.
• Adds unit and runtime SQL coverage for TRIM, LPAD, and RPAD.
• Fixes the failing runtime test by using the repository's actual ascii2str(int) function name instead of the nonexistent ascii2string(int) signature.

MySQL behavior comparison

Case	Result
Any argument is NULL	NULL
Target length is negative	NULL
Target length is zero	empty string
Target length is shorter than the input	input truncated by characters
Pad string is empty and padding is needed	original input
Multibyte/emoji input	counted by Unicode code point

Verification

• JDK 8: mvn -B -e -Pjdk8 -Dtest=PadTest,TrimTest -Dmaven.javadoc.skip=true install - 5 tests passed; Checkstyle and RAT passed.
• JDK 11: mvn -B -e clean test -Pjdk11 -Dtest=PadTest,TrimTest - clean compilation of 250 main and 47 test sources; 5 tests passed; Checkstyle and RAT passed.
• The corrected TRIM SQL query passes Calcite planning locally. Full runtime execution is covered by the Linux CI because the local Windows BRPC shutdown selects a VPN adapter address.

[914zzzz]

#789

[mingcheng]

Please check and fix the CI status before merging.

[914zzzz]

I rechecked both failed CI runs and found the actual cause: the new TRIM runtime test called ascii2string(9), but GeaFlow registers this function as ascii2str(int). Both JDK 8 and JDK 11 therefore failed during Calcite function resolution before TRIM executed.

Commit abd591a7 corrects the SQL test. I also removed the duplicate TRIM UDF registration because GeaFlow already supports TRIM through SqlStdOperatorTable.TRIM, and aligned negative LPAD/RPAD lengths with MySQL by returning NULL.

Local verification:

• JDK 8: PadTest/TrimTest, 5 tests passed; Checkstyle and RAT passed.
• JDK 11: clean compile of 250 main and 47 test sources; 5 tests passed; Checkstyle and RAT passed.

The new upstream runs are waiting for maintainer approval (they are action_required, not test failures):

• JDK 8: https://github.com/apache/geaflow/actions/runs/30994964454
• JDK 11: https://github.com/apache/geaflow/actions/runs/30994964311

Please approve these workflow runs when convenient. After they complete, the PR will still require one approving review from a reviewer with write access.