revert(protocol): withdraw invalid publisher domain patch

[bokelley]

What changed

• remove the publisher_domain filter from the get_products schema
• remove the training-agent implementation and its tests
• replace the feature changeset with a minor withdrawal changeset
• document 3.1.3 as withdrawn and 3.1.2 as the supported patch
• exclude 3.1.3 from schema/compliance aliases in both the CDN Worker and Fly fallback while preserving exact immutable URLs
• exclude unpublished 3.2.0 artifacts from aliases and latest metadata until that release is tagged
• apply withdrawn/unpublished status consistently to protocol discovery

Why

The feature was incorrectly backported and published as 3.1.3. Adding an optional field is a minor protocol change, not a patch-eligible change under the stable 3.1.x release contract. The implementation also accepted publisher_domains[] on products even though the product schema explicitly permits only singular publisher_domain selectors.

This PR restores the schema, implementation, and tests to their state before #5867. It retains minor release classification so the existing main Version Packages PR remains on the 3.2.0 line rather than attempting another 3.1.x patch. Published 3.1.3 artifacts remain immutable and are handled separately as a withdrawn release.

Validation

• 290 server unit test files passed (4,395 tests; 30 skipped)
• schema validation passed (19 checks across 648 schemas)
• TypeScript typecheck passed
• git diff --check passed
• artifact CDN Worker tests passed (30 tests)
• schema/protocol routing integration tests passed (29 tests)
• Wrangler production-route dry run passed

Deployment note

After merge, deploy the cutover Worker with npm run deploy:cdn-artifacts-cutover using Cloudflare credentials for the account that owns the adcontextprotocol.org zone. Exact 3.1.3 artifact URLs remain available; the deploy only removes 3.1.3 from movable aliases and discovery defaults.

[aao-secretariat]

Argus is not auto-reviewing this PR because it modifies protected paths that require human review (.changeset/training-agent-publisher-domain-filter.md, .changeset/withdraw-publisher-domain-filter.md, static/schemas/source/core/product-filters.json). A human reviewer should review and merge this PR; Argus will resume on subsequent PRs once these changes land on main.

[aao-secretariat]

Argus is not auto-reviewing this PR because it modifies protected paths that require human review (.changeset/training-agent-publisher-domain-filter.md, .changeset/withdraw-publisher-domain-filter.md, static/schemas/source/core/product-filters.json). A human reviewer should review and merge this PR; Argus will resume on subsequent PRs once these changes land on main.