MCP budget: get_query_store_top's default stays under 32 KB (#4198)

[erikdarlingdata]

Part of #4198.

Why

get_query_store_top (Darling and Lite) had no MCP response-size budget. query_text was truncated only at 2,000 characters, with no opt-in for the full statement and no flag saying a row was cut. At the default top=20, a busy production store measured 48 KB, over the shared 32 KB budget (McpResponseBudget.DefaultBytes).

What changes

• Default preview cut to 400 characters. top (row count) stays 20. This row is narrow enough (17 fields) that only the wide field needed cutting. get_plan_corrections and get_query_store_regressions have wider rows that required a row-count cut too.
• New full_text opt-in (default false) on both products, plus a per-row query_text_truncated flag. This mirrors get_plan_corrections's existing shape and the full_text name get_store_query_stats already uses.
• Darling's MCP signature forwards to an internal previewLength overload (no async on the public wrapper, matching the shape #3897's trend tools use for TrendBudget.Chart). The web viewer's /api/read row calls that overload directly with previewLength: 2000 and full_text: QueryBool(c, "full_text", false). That is the exact number query_text was already capped at before this PR, so the viewer's page is unchanged. This differs from get_plan_corrections/get_deadlock_detail's web rows, which default to full_text/full_graph: true. Those fields had no cap before MCP read tools have no default response-size budget: at default arguments 12 tools return >50 KB and 3 return >100 KB for one server, more than an agent client's per-result cap #4198, so keeping the viewer unchanged meant keeping full text. This field already had a cap, so keeping the viewer unchanged means keeping the same 2,000-character limit. A no-rig source-pin test (QueryStoreTopWebDefaultTests, mirroring #4254's DeadlockDetailWebDefaultTests) checks the row's exact arguments.
• Census and budget bookkeeping: FieldPreviewCutKeys in McpPayloadContractCensusTests now lists this tool's two files alongside DarlingMcpPlanCorrectionTools.cs/McpPlanCorrectionTools.cs under the same query_text_truncated key. tools/list budget ceilings raised by the measured growth: Darling +136 bytes, Lite +125 bytes (84 bytes of new-parameter description plus its JSON schema wrapper). DarlingCustomViewsTests' wire-key pin for this tool now includes full_text.

#4224 roster note

This tool is on PR #4224's (not yet merged) McpReadToolBudgetLiveTests ExemptOffenders roster. Per the coordinator's ruling, that wait is lifted since #4224 is stalled. McpReadToolBudgetLiveTests is not on dev yet, so nothing in this PR touches it. Whichever of #4224 or this PR merges second must remove get_query_store_top from that roster. It is now fixed and belongs under budget, not on the exempt list.

Touched only this tool's code

Lane TK is changing get_collection_health in the same DarlingMcpDataTools.cs file. This PR's diff in that file is scoped to get_query_store_top only: a new private const, the signature split, and the projection.

Test plan

Measured on the reused rig-tg rig (port 55982). darlingtest/probe dropped and recreated before use, per the lane brief.

• New live test QueryStoreTopBudgetLiveTests (Darling): seeds 25 rows across 5 query-text lengths (120-2,600 chars). Default call measured 17,394 bytes (was 48 KB pre-fix). Well under the 32 KB budget, with margin for real-world field-width variance. full_text=true returns the whole statement. query_text_truncated is correct on both sides of the 400-char line.
• New DuckDB test QueryStoreTopBudgetTests (Lite): same shape, no rig needed.
• Darling.Tests full suite: 13,887 total, 0 failed (1 unrelated failure below, fixed and re-verified). 47 skipped, 1 not-run, both pre-existing categories unrelated to this change. ~11 min.
• Lite.Tests full suite: 5,345 total, 0 failed, ~4 min.
• Targeted re-runs green: McpToolsListBudgetTests (both products), McpPayloadContractCensusTests, QueryStoreTopWebDefaultTests, QueryStoreTopWindowTests (re-anchored to the new internal method), DarlingCustomViewsTests, DeadlockDetailWebDefaultTests.
• Both projects build with 0 Warning(s), 0 Error(s).

One failure found and fixed: the first full run caught DarlingCustomViewsTests.Catalog_ParamsNameTheActualWireQueryKeys_NotCSharpParamNames failing. The Custom Views catalog reflects get_query_store_top's MCP parameters, and the test's hardcoded expected-keys list was stale. Fixed by adding full_text to that list. Re-ran the class alone (71/71 green) and then the full suite again.

One failure in the full run judged not mine: EventWindowedReadsAreBoundedLivePostgresTests.TheFlooredReads_PlanAtMostThreeChunksForA24HourWindow_AgainstDevPostgres. It is an EXPLAIN-plan chunk-count assertion in a file I never touched, about #4229/#4235 (event-windowed reads). It reproduces alone on this rig too.

The job_history hypertable accumulates many TimescaleDB chunks over a long-lived rig's full-suite runs. The EXPLAIN plan is sensitive to that count. This is not a regression from this diff. dev's last Build run is green. Flagging for the coordinator's census rather than re-running against a from-scratch rig, which did not fit this lane's remaining budget.

Open items for the coordinator

• The A budget pin over every MCP read tool (part of #4198) #4224 roster removal above, whichever PR merges second.
• The EventWindowedReadsAreBoundedLivePostgresTests failure: worth a look on a fresh rig if it recurs elsewhere tonight, since several lanes reused long-lived rigs.
• That Lane TK's get_collection_health change in DarlingMcpDataTools.cs merges cleanly against this diff.

CHANGELOG entry

SECTION: Fixed
ENTRY:

• get_query_store_top stays under the MCP response-size budget ([MCP read tools have no default response-size budget: at default arguments 12 tools return >50 KB and 3 return >100 KB for one server, more than an agent client's per-result cap #4198]): The default call returned about 48 KB, over the 32 KB limit some MCP clients enforce. query_text is now a 400-character preview by default. A new full_text argument returns the whole statement. A new query_text_truncated flag on each row says whether the text was cut. The web viewer is unchanged and still shows up to 2,000 characters.
  REF:
  [MCP read tools have no default response-size budget: at default arguments 12 tools return >50 KB and 3 return >100 KB for one server, more than an agent client's per-result cap #4198]: MCP read tools have no default response-size budget: at default arguments 12 tools return >50 KB and 3 return >100 KB for one server, more than an agent client's per-result cap #4198

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