fix(docs): restore the Next build — REMOVED_PROVIDERS.md has no frontmatter (base-red #12581)

[diegosouzapw]

Summary

Fixes the Package artifact (npm pack policy) hard failure behind base-red #12581 on release/v3.8.51, plus the Tarball boot-smoke failure that cascades from it.

Root cause. source.config.ts feeds docs/reference/**/*.md into defineDocs(), and fumadocs-mdx's default frontmatter schema requires a title string. #12478 added docs/reference/REMOVED_PROVIDERS.md with no frontmatter block at all, so every production build dies:

Error: Turbopack build failed with 1 error:
./docs/reference/REMOVED_PROVIDERS.md
Error: [MDX] invalid frontmatter in docs/reference/REMOVED_PROVIDERS.md:
- title: Invalid input: expected string, received undefined

One missing block turns three release-green gates red at once — the build itself, check:pack-artifact, and the boot-smoke that is skipped for lack of a valid dist/.

Changes

1. docs/reference/REMOVED_PROVIDERS.md — add the frontmatter block, matching its sibling reference docs (title / version / lastUpdated). The document's content is untouched.
2. scripts/check/check-docs-frontmatter.mjs (new) — validate that every Markdown file fumadocs compiles carries a non-empty title, wired into check:docs-all. It reads the globs from source.config.ts instead of duplicating them, so adding a docs directory there cannot silently escape the check.

Why the gate, and not just the one-line fix

Nothing in the current gate set catches this: it is not a broken link, not a fabricated reference, not a docs-sync drift — it surfaces only as a generic Turbopack error at the end of a full production build. Cost of catching it here: milliseconds. Cost of catching it in CI: a full build, three red gates, and a blocked release branch.

Verification

$ npm run check:docs-frontmatter
[docs-frontmatter] OK — 122 compiled doc(s) carry a valid frontmatter title.

# mutation check — gate actually catches the regression
$ (frontmatter removed) && node scripts/check/check-docs-frontmatter.mjs; echo $?
[docs-frontmatter] FAIL — 1 doc(s) would break the Next build:
  - docs/reference/REMOVED_PROVIDERS.md: no frontmatter block (fumadocs requires a `title`)
1

$ npm run check:docs-all   # passes (doc-links, fabricated-docs, counts, env-sync)

A scan of all 122 docs matched by source.config.ts found this file to be the only violation, so the branch is not one fix away from the next identical failure.

Refs #12581. Regression from #12478.

[diegosouzapw]

Verification: the build is fixed end-to-end

Full production build on this branch:

✓ Compiled successfully in 31.8min
BUILD_EXIT=0

Same command on the branch tip without this change fails with:

Error: Turbopack build failed with 1 error:
./docs/reference/REMOVED_PROVIDERS.md
- title: Invalid input: expected string, received undefined

Red discrimination

The two red checks are inherited from the base, not introduced here:

Check	Verdict	Evidence
Fast Quality Gates	inherited	fails on all 7 open PRs against release/v3.8.51 (#12604, #12606, #12607, #12608, #12590, #12591 and this one)
Unit Tests fast-path (2/4)	inherited / flaky	the failing case is tencent-aistudio-web executor returns wrapper shape — TimeoutError: Direct response did not start within 30000ms, a live-network test that waited 60s. #12591 fails the same job on a different shard (3/4).

This diff touches one Markdown frontmatter block, one new gate script, and one package.json line — no production code, no tests. tests/unit/removed-providers-blocklist.test.ts, the only test that reads the edited doc, passes 11/11 locally.

Base-red context: #12581.