Backport fixes for AnalyzingInfixSuggester (#1259)

* Backport fixes from LUCENE-5889, LUCENE-7564, and LUCENE-7670

* Backport SearcherManager thread-safety from LUCENE-7564 (73f068e5033)

Force single-threaded access to the SearcherManager when performing
an acquire() or reassigning, to prevent race conditions between
Build(), EnsureOpen(), DoLookup(), GetSizeInBytes(), and Count.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* PR feedback and fix linter error of writer field name

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: paulirwin