feat(loadtesting): add Load Testing capability index (regression)

[sourabhd-cbu]

What

The regression variant of the Load Testing capability index (base_url = https://load-testing-rengg-lts.bsstag.com), now updated to:

1. Resolve the merge conflict against feat/capability-registry. The base advanced since this PR opened (prod index feat(loadtesting): add Load Testing capability index (prod) #388 merged, auth scheme feat(loadtesting): declare the HTTP Basic auth scheme in the index #391 merged). This branch now merges base cleanly and adopts the auth: { type: http, scheme: basic } block from feat(loadtesting): declare the HTTP Basic auth scheme in the index #391 — which resolves the earlier invoke-401 caveat (the registry now declares HTTP Basic).
2. Add agent guidance that fixes the tool-call efficiency issues QA found on the agent surface (LTAI-270) — identical to the prod companion, differing only in base_url.

Guidance changes (guidance/summary text only; no endpoint or schema changes)

• report: prefer narrow slices (metrics / errorCategory / slaOnly / sinceIso+untilIso / detail=per-txn) over refetching the full report.
• getLoadTest / trends / run-history: one call returns the whole result — reuse within a task instead of re-fetching.
• estimateLoadTestRunCost: always use it for cost/VU-hour math (models ramp-up) instead of computing by hand.
• compareLoadTestRuns: use it for run deltas instead of diffing two reports.
• getLoadTestRunStatus: a status question needs status only, not the report.
• summary: state delete/scheduling are not offered (point to the dashboard) so the agent stops searching to confirm the gap.

Notes

• Prod companion: feat(loadtesting): agent guidance to cut redundant tool calls (prod) #394 (identical guidance, prod base_url). This branch and the prod variant differ by exactly one line (base_url).
• Reg and prod are separate deployments — do not merge both into the same deployment.

Ref: LTAI-270.

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