Add tests for Bazel sourceroot inference

[jupblb]

Fixes #444.

Adds end-to-end tests for the sourceroot inference of -build-tool:bazel compilations (ScipTaskListener.inferBazelSourceroot), as requested in a review comment on #439.

BazelSourcerootTest drives the real compiler with file objects that mimic the ones constructed by Bazel's Java compiler: toUri() points into a sandbox directory while toString() renders a human-readable workspace path. The tests assert the observable outputs: shard locations under targetroot/META-INF/scip/ and Document.relative_path. Covered cases:

• a sandboxed two-file compilation where the sourceroot is inferred from the first compilation unit,
• sandbox and workspace layouts sharing only the file name,
• unrecognized file objects (toString() matching no known Bazel pattern), where inference falls back to the URI path.

Notes:

• javac wraps client-supplied file objects in ClientCodeWrapper.WrappedJavaFileObject, whose toString() would defeat the plugin's pattern matching. Real Bazel file objects are javac-internal classes and therefore trusted/unwrapped; the test file object opts out of wrapping via @ClientCodeWrapper.Trusted. Since that annotation is an internal API, compileTestJava in scip-javac now clears --release and adds the same --add-exports flags the test JVM already uses at runtime.
• Writing the tests exposed a crash in the previously untested fallback path: when the human-readable path equals the URI path (e.g. unrecognized file objects), absolutePath.subpath(0, 0) threw IllegalArgumentException. The inference now falls back to the filesystem root in that case, and the third test covers it.