feat(md-exports): Improve soft-404 recovery for agent .md requests

[sergical]

DESCRIBE YOUR PR

Agent-facing .md soft-404s (the docs.md_export.not_found metric, ~6.9k hits since July 20) fall into three classes that today all get the same generic "Page Not Found" markdown. This PR gives each class a response the agent can actually act on:

• platform-redirect (top missed path, ~330 hits/month): new md-overrides/platform-redirect.mdx explains that the page is a platform chooser with no content of its own and shows how to build /platforms/<platform>/<page>.md URLs directly, with platform and framework link lists rendered from the doctree.
• Redirected pages (e.g. /product/sentry-mcp.md): the md-exports catch-all route now consults both redirect tables (redirects.js and the middleware legacy list) and serves a "Page Moved" body pointing at the destination — the .md URL for internal targets, the URL as-is for external ones like mcp.sentry.dev. Pattern (:path*) sources are skipped for now; the new metric attribute will show whether they matter.
• Pages that exist but whose static export was missing (transient deploy-window gaps, e.g. the product.md burst on July 30): the route detects doctree hits and serves "Markdown Export Unavailable" with the HTML link and child pages, cached for 60s so a restored export takes over quickly.

The metric gains an outcome attribute (redirected | page_exists | unknown_path) so dashboards can separate agent-invented URLs from export-pipeline gaps. The metric name and existing attributes are unchanged.

Review notes: the middleware.ts diff is only export keywords on the two redirect arrays and the Redirect type — route.ts is the substantive change. Verified with 232 passing vitest tests (8 new covering redirect, external-destination, pattern-skip, and page-exists paths), clean tsc/ESLint/Prettier, and the override rendered through a mirror of the generate-md-exports MDX pipeline against the real doctree. On the Vercel preview, check /platform-redirect.md and /product/sentry-mcp.md.

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PRE-MERGE CHECKLIST

Make sure you've checked the following before merging your changes:

• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

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Project	Deployment	Actions	Updated (UTC)
develop-docs	Ready	Preview	Aug 6, 2026 12:19am
sentry-docs	Ready	Preview	Aug 6, 2026 12:19am

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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^{Reviewed by Cursor Bugbot for commit 83b7042. Configure here.}

[cursor]

Platform override is never generated

High Severity

platform-redirect.mdx has no static HTML artifact to override, so platform-redirect.md continues falling through to the generic not-found route.

 

^{Reviewed by Cursor Bugbot for commit 83b7042. Configure here.}

[cursor]

Synthetic doctree nodes appear as pages

Medium Severity

Exact doctree matches are treated as real pages without checking node.missing, so synthetic hierarchy nodes receive a false “Markdown Export Unavailable” response and page_exists metric.

 

^{Reviewed by Cursor Bugbot for commit 83b7042. Configure here.}