Fix plain text log output dropping trailing partial lines

[thswlsqls]

Part of #26739
Follow-up to #26871

AnnotatedLargeText.writeLogTo(long, OutputStream) neither flushes its PlainTextConsoleOutputStream line buffer nor back-tracks the returned offset, so a trailing partial line is withheld but still counted as delivered: a client resuming at that offset never sees it. #26871 fixed both in the sibling writeHtmlTo(long, Writer), adding lineBufferSize() for it, but not here.

Reached by tailing a running build: logText/progressiveText with Accept: multipart/form-data runs doProgressTextStreaming -> writeLogTo(long, Writer) (non-HTML) -> writeLogTo(start, new WriterOutputStream(...)) -> here, and that streaming branch bulk-writes to EOF instead of stopping at a line boundary, so the partial line stays buffered. Buffered progressiveText sets delegateToWriteLogTo=false and never reaches this method; Run/_api.jelly (lines 48, 61) documents both.

Testing done

No Jenkins instance available. Reviewer steps: run a build emitting text without a trailing newline (sh 'printf partial'), then poll curl -H 'Accept: multipart/form-data' "$JENKINS/job/$NAME/$N/logText/progressiveText?start=0" while it runs.

• New AnnotatedLargeTextPartialLineTest: 3 of 5 cases fail without the fix; all assert emitted bytes and the returned offset.
• JDK 21, build cache off: mvn -pl core -am test -Dtest='TaskActionTest,AnnotatedLargeTextPartialLineTest,LineTransformationOutputStreamTest,UrlAnnotatorTest' gave Tests run: 12, Failures: 0, Errors: 0, Skipped: 0.
• mvn spotless:check -pl core passed.

Screenshots (UI changes only)

N/A, no UI change.

Before

After

Proposed changelog entries

• Fix loss of trailing partial lines when tailing plain text console logs.

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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