fix: stop text line scanner at empty EOF

[lowgame]

What changed

TextLineScanner.Scan now stops when ReadString reaches EOF without returning any bytes. The regression tests cover empty input, newline-terminated input, and legitimate blank lines.

Reproduction

Before this change, scanning either an empty reader or "one\n" yielded a synthetic final empty line:

• "" produced []string{""} instead of no lines.
• "one\n" produced []string{"one", ""} instead of []string{"one"}.

The new regression test failed on both cases before the production change.

Root cause

bufio.Reader.ReadString returns ("", io.EOF) when no data remains, but Scan returned true for every EOF result. That made an empty EOF look like a real line.

Fix

Return true at EOF only when ReadString also returned bytes. This preserves a final unterminated line and empty lines represented by newline delimiters.

Verification

• go test ./core/iox -run 'TestScanner|TestBadScanner' -count=1 — pass
• go test ./core/iox -count=1 — pass
• go test -race ./core/iox -count=1 — pass
• go vet ./core/iox — pass
• gofmt -l core/iox/textlinescanner.go core/iox/textlinescanner_test.go — no output
• git diff --check — pass

A full go test ./... was attempted but did not complete on this host: an unrelated core/logx access-permission assertion fails when run as root, and later packages exhausted the available disk while compiling Kubernetes dependencies.