fix(tests): assert the truncation marker where it actually lands (#464)

test_tool_result_logged_truncated_with_debug scanned only the lines containing
"mcp tool:" for "...[truncated]". truncateForLog appends "\n...[truncated]", so
the marker is on the line *after* the event and that assertion could never pass
-- the result was being truncated correctly the whole time.

Now asserts on the event line plus the one following it. Deliberately not a
whole-log scan: with --debug, request/response body capture truncates through
the same helper, so a whole-log search would pass on a marker from a different
code path and stop testing this one.

Verified: both #464 tests pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ccLBbEaVVd4sJyUd5wyhA