Add reproducer for check raw file injection smoke-test flake

[bm1549]

What Does This Do

Adds OutputThreadsTest, a JUnit 5 reproducer for the dominant flake of check raw file injection in LogInjectionSmokeTest (31 of 41 reports in CI Visibility):

java.lang.IndexOutOfBoundsException: toIndex = 3
  at java.util.AbstractList.subListRangeCheck(...)
  at datadog.smoketest.LogInjectionSmokeTest.parseTraceFromStdOut(LogInjectionSmokeTest.groovy:416)

The test exercises the buggy path deterministically via a ChunkedInputStream that returns one chunk per read() call. Both tests pass today and will start failing once OutputThreads is fixed. That makes them a ready regression test for the fix.

Motivation

The proximate trigger is a partial-read bug in OutputThreads.ProcessOutputRunnable.run(): when rc.read(buffer) returns a chunk with no newline AND the inner loop has consumed no lines yet, the fall-through branch decodes the partial buffer and adds it to testLogMessages as if it were a complete line. Under CI load the OS pipe splits the child's System.out.println("THIRDTRACEID <traceId> <spanId>") write. The smoke test then parses a truncated "THIRDTRACEID 12345" chunk via split(" ")[1..2]. The Groovy range translates to subList(1, 3) on a 2-element list, hence toIndex = 3.

Prior fixes (#10920, #10999, #11075) ruled out the assertion bug, the BaseApplication timeout, and process liveness, but left the partial-read issue in place. This reproducer isolates that issue without touching the production code path, so the fix can be reviewed against a deterministic test.

Additional Notes

• The reproducer lives in dd-smoke-tests/src/test/java/, a new test source set for the parent module. Adding testImplementation(libs.junit.jupiter) makes the JUnit 5 API available at test-compile time; the JUnit Platform runner is already enabled by the project's default Gradle setup.
• Verified deterministic across 5 consecutive runs locally.
• A follow-up PR will fix OutputThreads and flip these tests to assert correct behavior.

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors

Note: Once your PR is ready to merge, add it to the merge queue by commenting /merge. /merge -c cancels the queue request. /merge -f --reason "reason" skips all merge queue checks; please use this judiciously, as some checks do not run at the PR-level. For more information, see this doc.