fix(registry): gate brand.json serving on source_type to prevent enriched data misrepresentation

[bokelley]

Closes #3528

The /brands/:domain/brand.json endpoint was serving Brandfetch-enriched entries as if they were brand-attested brand.json files. The response spread raw Brandfetch data (name, url, company, description, classification) at the root level of the response, injected a non-spec name field, and omitted $schema — producing 40+ validation errors against the brand.json schema, which has additionalProperties: false on all four variants.

Changes:

• Gate on source_type: only brand_json and community source types are served; enriched (Brandfetch) and any other non-attested type returns 404. Serving third-party commercial data under a brand-attested URL path is both a protocol correctness failure and a trust/consent violation.
• Community entries additionally require review_status !== 'pending' and a two-field structural shape check (presence of house+brands, house string, agents, brand_agent, or authoritative_location) before being served.
• Inject $schema: https://adcontextprotocol.org/schemas/v3/brand.json when the stored manifest lacks a well-formed HTTPS $schema value. Uses the /schemas/v3/ alias (released stable schema) per playbook schema URL conventions.
• Remove the erroneous root-level name injection — name is not a top-level field in any brand.json variant; its presence caused additionalProperties: false failures on every served document.

Non-breaking justification: Consumers currently receiving invalid enriched data will now receive 404. A 404 is strictly less harmful than invalid JSON that silently fails schema validation. The A2UI theming feature (brand-viewer.html) already has a graceful 404 fallback path and is unaffected. No schema changes.

Pre-PR review:

• code-reviewer: approved — raised two blockers (community review_status gate, $schema URL alias) and one nit ($schema pass-through validation), all addressed in the final diff
• ad-tech-protocol-expert: approved — confirmed additionalProperties: false on all four oneOf variants makes the root name injection a spec violation; confirmed 404 for enriched is protocol-correct; confirmed /schemas/v3/ is the correct alias for this released schema

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See #3121
for context.

Session: https://claude.ai/code/session_01PR6QXBoh1GW1qqrnz6jEkY

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