Fix incorrect position increment generated by FixedShingleFilter

[kkharbas]

Summary

FixedShingleFilter could emit shingles with an incorrect position increment when a route in the graph cannot form a shingle, but another route can. In that case, after reseting the base token, the position increment is set to 0 since its an alternate path. However this is incorrect if the previous route did not generate any token.

Example: For FixedShingleFilter("Secure Wi-Fi:2/Wi, Fi", 2) with Wi-Fi as base token, the first route does not yield token, the next route with Wi incorrectly uses position increment = 0

Fix

• Introduce pendingPosInc to accumulate position increments from base tokens that don't produce a shingle, so the next emitted shingle carries the full, correct increment.
• Added an end() override to fold any remaining pendingPosInc into the final position increment reported by TokenStream.end().
• Added a reset() override to clear pendingPosInc on reuse.
• Added two regression tests (testPositionIncrementAfterSkippedGraphToken and testFirstShinglePositionIncrementAfterSkippedGraphToken) that reproduce the bug and verify the fix.

Testing

• ./gradlew :lucene:analysis:common:test --tests "*.TestFixedShingleFilter" passes
• Existing FixedShingleFilter tests continue to pass unchanged