[Chore] Guard on-demand integration argument schemas and dispatch

[roomote-roomote]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

Strengthen on-demand integration argument regressions for the schemas introduced in #2237:

• Compile the complete serialized schemas with Ajv, exercising recursive references with nested objects, arrays, nulls, and other JSON primitives.
• Exercise sandbox MCP registration, wire validation, and argument dispatch over an in-memory MCP connection. Verify outer null/omission becomes an empty argument object and invalid argument types never reach dispatch.
• Verify the generated Fast tool's JSON Schema and Zod parser agree on accepted inputs.
• Correct test descriptions that implied local schema checks proved live OpenAI acceptance.

Why this change was made

Checking only the presence of an object branch does not prove recursive references resolve or that arguments survive runtime validation. These checks cover those gaps without changing the production schema or transport contract.

Impact

Test-only change; no production behavior changes and no claim of a newly reproduced or fixed null-only provider failure. The local OpenCode 1.18.10 schema transform preserved the object branch for both the earlier unconstrained record and the current recursive schema. Actual provider acceptance and deployment-specific behavior remain outside these tests.

The targeted worker and Fast suites pass (15 tests). Package typechecks and the full pre-push lint, typecheck, and knip gates pass. External integration calls and live-provider requests were not exercised.

[roomote-community]

No code issues found. See task

_{Reviewed 3aac1ff}