Allow forked executor to finish with interactive stdin

[efegokdemir]

Summary

Fixes #50 by allowing a forked process to finish without waiting for a caller-supplied stdin stream to reach EOF.

Changes

• Wait for the child process and the stdout/stderr pumps independently of the stdin pump.
• Close the child stdin stream after process completion.
• Add a regression test using a non-EOF stdin stream.

Testing

• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home PATH="$JAVA_HOME/bin:$PATH" mvn -pl maven-executor -Dtest=ForkedMavenExecutorTest test — 16 tests passed, 1 skipped.
• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home PATH="$JAVA_HOME/bin:$PATH" mvn -pl maven-executor spotless:apply — passed.
• Checkstyle and Apache RAT — passed.
• git diff --check — passed.
• Full module test run executed; the forked and toolbox suites passed, while the existing embedded color-output test defaultFs4CaptureOutputWithForcedColor failed in this environment because ANSI output was absent.

AI assistance was used; I reviewed the complete change and validation results.