fix(probes): align private rendezvous health fallback contract

[zoeyrose]

Summary

Align Observatory with the metaserver rendezvous-health v1 fallback contract and keep private-source failures diagnosable without exposing private response bodies.

Implementation / behavior

• Accept versioned no_usable_observation responses with reason: malformed_observation as Unknown observations, while retaining no_observation.
• Persist the normalized fallback reason and accept it when reading stored observations.
• Classify a non-JSON response Content-Type as source_invalid_headers and bounded JSON/UTF-8/schema failures as source_invalid_payload; retain fail-closed unauthorized/unavailable handling.
• Keep diagnostics bounded and free of rendezvous identifiers, tokens, or raw source payloads.
• Add synchronized producer-contract fixtures and tests for fresh evidence without a canary, malformed/stale/no-observation fallbacks, unauthorized/unavailable responses, malformed headers/content, and privacy.

Validation

• npm run check
• npm run build
• npm run deploy:dry-run
• Focused Observatory and metaserver contract tests plus final complete-diff review.

Limitations / follow-up

Production deployment and verification of the next scheduled observation require maintainer/operator approval and the configured Cloudflare bindings. This delivery does not mutate production state.

Closes #26

[cloudflare-workers-and-pages]

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Status	Name	Latest Commit	Updated (UTC)
❌ Deployment failed View logs	atrinik-observatory-probes-preview	f0dd038	Aug 27 2026, 07:51 AM

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