SOLR-17841: Add multithreaded search benchmarks for baseline comparison

[arup-chauhan]

This PR adds benchmark coverage for multithreaded search as a baseline measurement for SOLR-17841.

Scope is intentionally limited to a single benchmark file. No production code is changed. The goal is to establish reproducible, comparable numbers before any fix is attempted.

Changes

Modified ExitableDirectoryReaderSearch.java in solr/benchmark:

• Added @Param({"false", "true"}) boolean multiThreaded to BenchState. JMH will now cycle through both normal and multithreaded mode for every benchmark automatically.
• Updated createInitialParams() to accept BenchState and conditionally set multiThreaded=true via CommonParams.MULTI_THREADED.
• Added a new @Benchmark method testRangeQuery querying id:[0 TO 250000], a numeric range covering half of the 500k indexed documents, targeting the specific query type reported as ~5x slower under multithreaded search.

Validation

• ./gradlew :solr:benchmark:compileJava
• ./gradlew tidy
• ./gradlew check -x test

[arup-chauhan]

Hey guys!
Thanks for your patience while I worked through this.
This PR establishes the benchmark baseline for SOLR-17841. Would love your eyes on it when you get a chance.

CC: @epugh @renatoh