Add WorkbookEvaluator context-aware fuzz target

[vishalcoc44]

This PR adds a context-aware fuzz target for the Apache POI WorkbookEvaluator engine to the poi-fuzz module.

This fuzzer was developed as part of Google's OSS-Fuzz integration (see google/oss-fuzz#14987) and is being upstreamed

Technical Approach

• Context-Aware Strategy: Pre-populates a 20×20 Excel data grid with a diverse mix of numbers, negative values, strings, booleans, and error codes so that referential formulas (SUM, VLOOKUP, INDEX, MATCH, etc.) exercise real evaluation paths instead of fast-failing on empty references.
• Guided Input Generation: Uses a dictionary of 60 complex Excel function signatures to steer the fuzzer toward deep calculation branches in the WorkbookEvaluator.
• Coverage Impact: In local testing, this increased code coverage within the WorkbookEvaluator from 19 lines to over 2,200 lines (~11,700% increase).
• Security Alignment: Expected exceptions (FormulaParseException, NotImplementedException, etc.) are suppressed while internal logic flaws (NPE, OOB) bubble up as findings.

Placement

File added to poi-fuzz/src/main/java/org/apache/poi/fuzz/WorkbookEvaluatorFuzzer.java

[vishalcoc44]

@centic9

[vishalcoc44]

@centic9