fix: use DEP codes for deprecation metadata slugs

[Thenightrevenge]

Description

This updates metadata slug generation for the deprecations page so headings like DEP0001: http.OutgoingMessage.prototype.flush use the short DEP0001 anchor instead of the full heading text.

It keeps normal slugging for DEP-style headings outside the deprecations page and adds deterministic suffixes if the same DEP code appears more than once.

Validation

• git diff --check
• Added unit coverage for deprecations-page DEP slugs, duplicate DEP slugs, and normal slug behavior outside the deprecations page.
• Attempted focused Node test: node --test --experimental-test-module-mocks src/generators/metadata/utils/__tests__/parse.test.mjs, but the local sparse clone has no installed npm dependencies and stopped at ERR_MODULE_NOT_FOUND: Cannot find package 'unist-builder'.

Related Issues

Fixes #790.

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run node --run test and all tests passed.
• I have check code formatting with node --run format & node --run lint.
• I've covered new added functionality with unit tests if necessary.

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Project	Deployment	Actions	Updated (UTC)
api-docs-tooling	Ready	Preview	Jun 3, 2026 2:28pm

[cursor]

PR Summary

Low Risk
Localized metadata slug logic for one doc path, covered by new unit tests; no auth, runtime API, or broad refactors.

Overview
On the deprecations API doc only, heading metadata slugs now use the DEP#### code from headings like DEP0001: … instead of a slug derived from the full title. Duplicate codes on the same page get suffixed (DEP0001-1, etc.); non-matching headings on that page still use the normal slugger.

Other API pages are unchanged—DEP#### headings there still get the usual slugified anchors (e.g. dep0190-some-section-heading). A shared DEPRECATION_HEADING_REGEX drives the behavior, with unit tests covering all three cases.

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