Fix addMultiTermClauses: preserve per-field SHOULD when a clause isn't a bare TermQuery (#16441)

[OlivierJaquemet]

Fixes #16441

Problem: QueryParserBase#addMultiTermClauses decides whether to pass a nested BooleanQuery's clauses through unchanged, or flatten them under a single forced Occur (MUST under AND_OPERATOR), based on whether every clause is a bare TermQuery. This check is too narrow: a BoostQuery-wrapped term (or any other non-TermQuery leaf — PrefixQuery, PhraseQuery, etc.) fails it, causing MultiFieldQueryParser's per-field SHOULD disjunction to be incorrectly flattened and forced to MUST on every field independently — turning "any field may match" into "every field must match."

Example: with a per-field boost configured and AND_OPERATOR, MultiFieldQueryParser.parse(QueryParser.escape("hello !")) produces +(field1:hello)^2.0 +field2:hello instead of the expected (field1:hello)^2.0 field2:hello.

Fix: the real distinguishing signal is not the leaf's type but whether q's direct clauses are themselves nested BooleanQuery instances (= several term positions that must be joined per the default operator — flatten+force is correct there) versus any other leaf type (= field alternatives for a single term position, which must be passed through unchanged regardless of leaf type). One-line change:

- if (!(clause.getQuery() instanceof TermQuery)) {
+ if (clause.getQuery() instanceof BooleanQuery) {

Testing: added/updated unit tests covering OR/AND × boosted/unboosted × single-term/multi-term combinations, plus a PrefixQuery regression case, to confirm the fix doesn't disturb the existing multi-term AND-joining behavior (which relies on the same flatten branch and must stay intact).