[apache-poi] Add targeted FormulaParser fuzzer

[vishalcoc44]

This PR adds a targeted fuzzer for the Apache POI FormulaParser (org.apache.poi.ss.formula.FormulaParser).

Logic Crash Identified

During local stress-testing of this fuzzer, I identified a logic flaw in the "Structured Reference" state machine.

• The Bug: Prematurely terminated nested brackets (e.g., [[ or [[[]) cause the parser to increment its internal pointer beyond the string bounds.
• The Result: java.lang.RuntimeException: Parsed past the end of the formula, pos: 3, length: 2, formula: [[
• Status: Confirmed in Apache POI 5.2.3.

Implementation Details

• Target: FormulaParser.parse() (High complexity, high value).
• Hardening: Includes POIFuzzer.adjustLimits() and broad exception handling to ensure long-running stability.
• Optimization: Added a dedicated .dict file containing Excel formula tokens (Structured References, standard functions) to guide the fuzzer into deeper logic branches.

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Verification Plan (For Maintainers)

To verify the crash found by this fuzzer:

1. Run the included FormulaParserFuzzer.
2. It will immediately identify the [[ crash.
3. Standalone reproduction: FormulaParser.parse("[[", workbook, FormulaType.CELL, -1).

[github-actions]

vishalcoc44 is a new contributor to projects/apache-poi. The PR must be approved by known contributors before it can be merged. The past contributors are: centic9, centic9-dt, arthurscchan, aschaich