changelog: source bundle entries from the CDN with a per-product registry

[cotti]

Note

Stacked on changelog_directive_s3. Base this against that branch and merge it first; the diff shown here is only this change.

Summary

Source the individual changelog entries that make up a bundle from the public CDN by default (scoped to the bundle's product), instead of the local docs/changelog/ folder. This makes bundles reflect what was actually published to S3 — i.e. with private references scrubbed — and decouples bundle creation from a checkout that happens to hold every entry.

The bundle filter is content-based: whether an entry belongs in a bundle depends on the products / prs / issues fields inside the YAML, not on the file name. CloudFront has no ListObjects, so we publish a small per-product index on upload and the client enumerates → fetches → filters.

What changed

• Per-product entry registry: upload now writes {product}/changelog/registry.json alongside the existing bundle registry (RegistryBuilder / RegistryKey); the scrubber key allow-list recognizes it.
• CDN entry sourcing: new CdnChangelogEntryFetcher downloads the entry index and each entry; CDN base resolution is centralized in ChangelogCdn (shared with the changelog directive's cdn: mode).
• Opt-out + safe fallback: bundle.use_local_changelogs: true forces the local folder, and we fall back to local automatically when no concrete product can scope the per-product CDN fetch.
• --plan cdn_url: changelog bundle --plan now emits cdn_url ({base}/{product}/bundle/{file}) so CI can poll for the scrubbed bundle. docs/cli-schema.json regenerated.
• No silent gaps: a registry-listed entry that hasn't propagated to the CDN yet is retried with short backoff + cache-busting (defeats a CloudFront-cached 404); a persistent miss fails the bundle rather than shipping an incomplete release.

Verification

• dotnet format --verify-no-changes: clean
• dotnet publish -c Release (docs-builder): 0 trim/AOT warnings
• Elastic.Documentation.Configuration.Tests (409/409) and Elastic.Changelog.Tests (732/732): pass
• cli-schema.json matches -- __schema output

Test plan

• upload publishes {product}/changelog/registry.json and individual entries
• bundle (default) fetches scrubbed entries from the CDN and applies the content filter
• bundle with use_local_changelogs: true uses the local folder
• bundle --plan surfaces a correct cdn_url
• A not-yet-propagated entry recovers via retry; a persistently missing entry fails the run

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