Fix FixedShingleFilter emitting a leading 0 position increment (GITHUB#14137)

[vismaytiwari]

Description

Fixes #14137.

With a valid analyzer chain — StandardTokenizer → WordDelimiterGraphFilter(GENERATE_WORD_PARTS | GENERATE_NUMBER_PARTS | CATENATE_NUMBERS) → FlattenGraphFilter → FixedShingleFilter — indexing input such as 555,0 throws IllegalArgumentException: first position increment must be > 0 (got 0). This is the same chain Elasticsearch's search_as_you_type uses.

FixedShingleFilter took a shingle's position increment from the emitting base token. But when earlier base positions produce no shingle (their graph routes are too short to fill the window) their increments were dropped, and a base reached as a graph alternative can itself carry a posInc of 0 — so the stream's first shingle could be emitted with posInc=0, which the indexer rejects.

For 555,0 the flattened graph feeding the filter is "5550"(posInc=1), "555"(posInc=0), "0"(posInc=1). The "5550" base can't form a bigram and emits nothing, so the first emitted shingle "555 0" is based at "555" (posInc=0) and inherited that 0.

The fix accumulates the base-position increments across bases that emit nothing and applies the total to the next emitted shingle (0 for further graph routes stacked at the same base). So "555 0" now carries 1 + 0 = 1. Normal streams are unchanged: a single base still contributes its own increment, and stacked graph routes at the same base still get 0.

Added TestFixedShingleFilter#testGraphInputDoesNotEmitLeadingZeroPositionIncrement, which builds the reported chain over 555,0. It fails on main (leading posInc=0) and passes with this change. Full TestFixedShingleFilter and ./gradlew tidy are green.

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