docs(integrations): document testing API

[EmersonBraun]

What

• Document the 2 previously undocumented callable symbols in @agentskit/integrations/testing.
• Shrink the JSDoc coverage baseline for this subpath.

Why

• Make the pure action and trigger validators easier to use from the testing entry point.

How

• Add parameter and return documentation based on the validators' implementations.
• Add the required patch changeset. No runtime logic or public signatures changed.

Validation

• pnpm --filter @agentskit/integrations lint
• pnpm --filter @agentskit/integrations test (24 files, 157 tests passed)
• pnpm --filter @agentskit/integrations build
• node scripts/check-jsdoc-coverage.mjs --update (32/116 documented; the remaining 84 are root entry point symbols handled by PR docs(integrations): document public API #1812)
• pnpm check:public-api-snapshot (24 packages, 74 subpaths, 1,955 symbols; unchanged)
• Repository pre-push quality gates passed.

[EmersonBraun]

Superseded by #1816, which consolidates the integrations testing subpath documentation with the remaining batch packages.