ci(spring-matrix): Replace sed hacks with targeted Gradle builds (#5397)

* ci(spring-matrix): Replace sed hacks with targeted Gradle builds

Remove the sed-based Android module exclusion from settings.gradle.kts
and build.gradle.kts in the Spring Boot matrix workflows. This is
unnecessary because `org.gradle.configureondemand=true` ensures Gradle
only configures projects needed for the requested tasks.

Replace the broad `./gradlew assemble --parallel` with a single
targeted Gradle invocation that builds only the specific artifacts
needed (shadowJar/bootJar/war + OTel agent). Remove redundant
`--build "true"` from test runner invocations since artifacts are
already built.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* ci(spring-matrix): Include testClasses in initial build

Add testClasses tasks to the single Gradle invocation so test
sources are pre-compiled. The subsequent systemTest Gradle calls
then only execute tests without needing to compile anything.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* ci(spring-matrix): Remove redundant --parallel flag

Already set in gradle.properties.

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

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>