Fix #1275: stop using PrintWriter in AbstractLineAggregatingHandler to avoid swallowing IOExceptions

[elharo]

Summary

fixes #1275.

AbstractLineAggregatingHandler.addToArchive wrapped the aggregation output stream in a PrintWriter. By design, PrintWriter catches IOException internally and only records it via checkError(). Any write failure during aggregation was therefore silently swallowed, and a truncated or empty aggregated file was added to the archive without any error surfacing.

Changes

• Replaced PrintWriter with OutputStreamWriter in AbstractLineAggregatingHandler.addToArchive. Write failures now propagate as IOException and are wrapped in an ArchiverException by the existing catch block.
• Added a small protected seam newAggregationOutputStream(Path) so the write target can be replaced in tests.
• Added regression test AbstractLineAggregatingHandlerTest.addToArchiveShouldPropagateIOExceptionWhenAggregationWriteFails that injects an OutputStream which fails on write and asserts that addToArchive throws ArchiverException.

The test fails on the old code (PrintWriter swallows the exception, nothing is thrown) and passes with the fix.

DefaultMessageHolder also uses PrintWriter, but it wraps an in-memory StringWriter which cannot throw IOException; it is left unchanged (and Throwable.printStackTrace requires a PrintWriter).

Verification

• mvn verify: BUILD SUCCESS, 269 tests pass (1 new regression test).
• Behavior of the success path is unchanged: lines are written followed by the platform line separator, identical to the previous println behavior.