Fix unitTest task not running the test source set

[loggk]

Description

./gradlew unitTest runs 1 test class instead of 170 — the tests under src/test/java are never executed, locally or in CI.

unitTest is created with task<Test>("unitTest"), which has no testClassesDirs or classpath. The only place they get populated is the JDK-21 block, which appends to that empty base:

tasks.named<Test>("unitTest") {
    testClassesDirs += java21.output.classesDirs
    classpath = sourceSets["java21"].runtimeClasspath
}

So the task resolves to the java21 source set alone. This is a regression from #968, where those two lines were moved off tasks.test — whose testClassesDirs the java plugin populates, making += correct — and onto unitTest/integrationTest, which have no base.

Changes

Set testClassesDirs and classpath on unitTest from the test source set, so the JDK-21 block appends to it rather than defines it.

./gradlew :java-client:unitTest on JDK 21 (Temurin 21.0.11), counted from java-client/build/test-results/unitTest/:

	classes	tests	failures
before	1	1	0
after	170	426	0

All 426 pass, so no other change is needed to re-enable them. RequestOptionsTest from src/test/java11 still runs, and the excludeTestsMatching filter still keeps integTest classes out.

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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