fix(webapp): preserve log search batches with invalid JSON escapes

Prevent log search batches from being lost when a message contains an
invalid JSON escape. Search inserts now use the same lenient escape
setting as source inserts and share the existing bounded
sanitize/row-skip recovery, so an unrepairable row does not discard its
neighbors.

Drop counters start at zero for each reason, and recovered batches count
only skipped rows as dropped.

Mono-RevId: 353071db24c77a392baaa7cb96340f6ab64d8e3e