JSON extractor: application/json falls back to HTML (#168 follow-up)

[gregoryfoster]

Background

#168 (slice 2) made content_media_type drive extractor dispatch via ServiceRegistry.get_extractor (src/core/registry.py). Dispatch is total: any essence not explicitly mapped falls back to the HTML extractor.

application/json has no dedicated extractor, so JSON targets are currently run through HtmlExtractor. Archiver's declared content-kind family already includes json (jsonpath → json), but Watcher has no JSON extractor to match.

Ask

Add a JSON extractor (e.g. canonicalize/pretty-print + structural chunking, or JSONPath-aware extraction aligned with Archiver's jsonpath algorithm) and register it:

• src/core/extractors/ — new JsonExtractor (mirror to /home/exedev/archiver/src/core/extractors/ per the mirrored-content-acquisition policy).
• src/core/registry.py — map application/json (and likely application/*+json) → JsonExtractor.
• Tests: routing (application/json → JsonExtractor) + extraction behavior.

Notes

• HTML fallback is non-crashing today, so this is an enhancement, not a regression.
• Consider application/*+json (vendor JSON) essence handling at the same time.
• Related: magic-byte/extension hardening and drift detection were also deferred from Auto-detect content type from URI response; evaluate MIME vs enum #168.