addToolOutput retains the preliminary flag when setting a final tool result

Finding: In the vercel/ai issue #22595, the reporter says that on ai@7.0.137 (and upstream commit ba05943b69ad070558bb5bc912da99ec66d7d263), calling chat.addToolOutput() on a tool part that already holds a preliminary output sets the final output value but leaves preliminary: true. As a result, lastAssistantMessageIsCompleteWithToolCalls() returns false, and apps that use it for sendAutomaticallyWhen do not continue automatically (https://github.com/vercel/ai/issues/22595). Reported reproduction: a node script with a stub chat state holding a tool-task part in output-available state with output 'working' and preliminary true. After addToolOutput with output 'finished', the part has the new output but still preliminary: true, and the completeness assertion fails. It was reported on Node.js v26.7.0 and makes no network requests (same issue URL). A contributor-authored comment on the issue says the bug was reproduced on the same commit with ai@7.0.137. It also says the check covered static and dynamic tools in stored messages and active responses. Per that comment, final output-error states also kept the stale preliminary flag, and active final results produced zero automatic continuations instead of one (https://github.com/vercel/ai/issues/22595#issuecomment-6101867538). That comment references a reproduction PR (https://github.com/vercel/ai/pull/22608), which this record treats only as reproduction material, not as a fix. Suspected cause (reporter hypothesis, echoed by the contributor comment): addToolOutput in packages/ai/src/ui/chat.ts spreads the existing tool part into the new state without resetting preliminary, and offers callers no option to clear it. The stream processor, by comparison, takes the flag from each incoming chunk. The reporter proposes, untested, removing the stale flag on the transition to output-available and output-error. Status: no maintainer confirmation, no fix commit, and no release containing a fix appear in the supplied sources. No workaround is documented. Limitations: this is an external report with a synthetic reproduction and no KFA reproduction; comment coverage is partial; the cause is not maintainer-confirmed. Related #21083 is cited by the reporter as having changed the helper predicate, which is a different aspect from this state transition.