fix(ci): bound the forgotten-sibling report so an advisory step stops failing the job

[diegosouzapw]

"Fast Quality Gates" is currently red on every open PR against release/v3.8.51. The failing
step is forgotten-sibling-tests — which is explicitly advisory; its own output says
"Report-only calibration: these findings do not fail the job." — yet it exits 1.

Why

When a diff touches a hub module (open-sse/config/providerRegistry.ts in the current reds), the
analysis walks every import edge in the repo and multiplies each consumer by its candidate tests.
The cross-product reaches millions of rows, and lines.join("\n") then exceeds V8's maximum string
length. The throw lands in main()'s catch, which exits 1 — so an advisory report takes the whole
job down, on PRs that have nothing to do with it.

Measured with a synthetic hub cross-product against the current script:

findings	result
3 000 000	a 435 MB report string (no throw, but absurd)
4 500 000	Invalid string length — the CI failure, verbatim

With this change the same 4 500 000 findings render as 27 KB.

What changed

Only the enumeration is bounded — never the signal:

• the header keeps the exact totals (Unallowlisted findings: 4500000 in the probe above);
• at most 200 rows per section are listed, followed by a line naming how many were withheld;
• the JSON artifact gets the same treatment (5 000 items per array) plus an explicit totals
  object — JSON.stringify would throw on the same input for the same reason;
• markdown() is exported so the bound is testable without a CI-sized diff.

No change to the analysis, the allowlist, or what counts as a finding.

Note

This is the second base-red in a row from the same family: #13875 fixed the env/docs contract, and
this one clears the remaining always-red gate. Both are infrastructure, not product defects.