ci: optimize pull request checks

[bladehan1]

What does this PR do?

• Run the existing multi-platform PR Build jobs for qualifying pushes to develop.
• Replace six hand-written Gradle User Home cache steps with gradle/actions/setup-gradle@v6.
• Accept PR titles in both type(scope): description and type(scope):description form while continuing to reject an empty description.
• Remove two tracked 0-byte Java placeholder files from chainbase.

Why are these changes required?

The Gradle cache configuration is duplicated across platform jobs and requires separate cache paths, keys, and restore keys. Using the official Gradle setup action centralizes this behavior and reduces workflow maintenance.

Running the platform matrix after a push to develop validates the exact integration-branch commit in addition to the pull request merge ref. The title-rule adjustment avoids rejecting an otherwise valid conventional title solely because there is no space after the colon. The deleted Java files contain no declarations or behavior.

This PR has been tested by:

• YAML AST parsing for .github/workflows/pr-build.yml and .github/workflows/pr-check.yml
• Positive and negative PR-title regular-expression cases
• git diff --check HEAD^ HEAD
• ./gradlew -g /private/tmp/java-tron-gradle-home :chainbase:compileJava --no-daemon

Follow up

• Monitor the first PR and develop workflow runs to confirm the intended Gradle cache read/write scope and reuse behavior.
• Review whether the Debian coverage artifact should be uploaded for develop push events when the coverage gate is not scheduled.

Extra details

• Related issue: [Feature] Optimize pull request checks and task caching bladehan1/java-tron#46
• This PR does not change node runtime, consensus, state, database, RPC, configuration, or protocol behavior.