test(schema): add discovery cache coverage

[IFAKA]

Summary

• Adds focused unit coverage for schema discovery cache reads, writes, and expiry handling.
• Verifies FetchDiscovery uses a fresh local cache without making a network call.
• Keeps production behavior unchanged.

Why

pkg/schema previously had no test coverage, while discovery caching is important for avoiding unnecessary discovery document network calls. This is a small, low-risk coverage improvement that does not require Google Health credentials or live API access.

Validation

• go test ./pkg/schema -v
• go test ./pkg/schema -cover — 40.6% coverage
• go test ./...
• go vet ./...
• go build -o ghealth .

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[IFAKA]

This is ready for review. I do not have permission to request reviewers directly on this repo, but based on the repo activity @asmitapoddar looks like the right maintainer to route this. Could someone from the project please route/review when available?