Add `other_count` Rollup to Grouped Aggregate Responses

Why

Grouped aggregate responses (group_by and group_by_time) return the top-N groups ordered by count. Without a rollup field, callers cannot tell whether the top-N is a complete picture of the data or a truncated subset. A model or agent that trusts a limited facet list without knowing the tail size can draw incorrect conclusions (e.g. "these are all the senders" when 40% of records fell into groups beyond the limit). A second aggregation call to compute the tail is extra latency and an unnecessary round trip.

What Changes

• Every grouped aggregate response now includes an other_count integer: the sum of counts for all groups/buckets beyond the limit. Zero means all groups fit; a positive value signals truncation and quantifies the tail.
• other_count is emitted for both group_by (scalar top-N facets) and group_by_time (calendar time-bucket series) when a grouping dimension is present. It is omitted for ungrouped (scalar) aggregations.
• The field is present even when no truncation occurs (other_count: 0) so callers can use its presence as a reliable indicator that the response is a grouped response, not just a heuristic "check if groups.length == limit".
• The MCP aggregate tool description is updated to mention other_count so models know to check for truncation without a second call.
• The MCP summarizeAggregate helper surface the other_count value in the text summary when present.

Capabilities

New Capabilities

• None.

Modified Capabilities

• data-query-api: the aggregate endpoint's response contract gains other_count on grouped responses.
• mcp-adapter: the aggregate tool description and text summarizer surface other_count so models can detect top-N truncation in-band.

Impact

• Affected files: reference-implementation/server/records.js, packages/reference-contract/src/public/index.ts, packages/mcp-server/src/tools.js.
• Additive-only change: other_count is a new response field; no existing field is removed or renamed. Old callers that ignore unknown fields are unaffected.
• No REST query parameter changes, no storage changes, no grant-semantics changes.
• Affected tests: reference-implementation/test/aggregate-time-buckets.test.js (3 new + 2 inline assertions in existing tests), packages/mcp-server/test/aggregate-tool.test.js (existing suite still passes; description change covered by the description-teaches test).