fix(o11y): parse Analytics Engine aggregate strings

[jeanduplessis]

Summary

Parses Analytics Engine aggregate strings before validating Gastown health metrics.

Why this change is needed

Cloudflare Analytics Engine serializes SUM(...) values as strings in JSON. The o11y worker expected JSON numbers, so every Gastown health evaluation failed schema validation after the cardinality fix reached production.

How this is addressed

• Convert non-empty aggregate strings to numbers before strict validation.
• Continue accepting JSON numbers and nullable aggregates.
• Reject empty, non-numeric, negative, and non-finite aggregate values.
• Exercise production-shaped string aggregates in the main query test.

Human Verification

• O11y test suite: 15 files and 144 tests passed.
• O11y TypeScript typecheck passed.
• Independent logic and API-contract reviews found no issues.
• Production Cloudflare logs confirmed SUM(...) fields arrive as strings.

Reviewer Notes

Human Reviewer Flags

No notable items for human review beyond what the summary covers.

Code Reviewer Agent

Code Reviewer Notes

• Parsed output remains number | null, preserving downstream metric contracts.
• Validation rejects malformed numeric strings instead of using permissive coercion defaults.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• services/o11y/src/alerting/gastown-health-query.ts
• services/o11y/test/gastown-health-query.spec.ts

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_{Reviewed by gpt-5.4-20260305 · Input: 41.1K · Output: 5.4K · Cached: 309.8K}

_{Review guidance: REVIEW.md from base branch main}