test: cover fetcher gaps

[beanscg]

Summary

• add httptest coverage for dataset API fetching, dataset README parsing, dataset/model tree pagination, and model search
• add table-driven tests for markdown front-matter extraction and string-slice normalization
• fix splitFrontMatter so the closing --- marker is stripped from the returned body

Issue

Addresses part of #25.

Verification

go test ./internal/fetcher
go test ./...

Both pass locally.