Read tools (getPage, getPageJson, getNode, diffPageVersions,
exportPageMarkdown) return whole pages with no size cap. Their outputs
were stored verbatim in metadata.parts and the tool_calls column, and
metadata.parts is replayed to the provider on every later turn via
convertToModelMessages. After reading a couple of large pages the prompt
grew by full page bodies each turn — rising token cost, latency and DB
row size.
Add compactToolOutput(): a pure, recursive, size-bounded compactor used
in assistantParts() and serializeSteps(). It preserves the value's kind
and small scalar fields (id/title/pageId, which the client reads to build
citations on reload) while truncating long strings, capping long arrays
with a marker, and collapsing subtrees past a depth limit. Small outputs
are returned unchanged by identity. Tool inputs are left intact so
replayed tool_use arguments keep their object shape.
Compaction runs only at persistence time (onFinish/onAbort), so the live
stream and the current turn's multi-step reasoning still see full bodies.
Add unit tests for compactToolOutput.
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