[kayx23]

Dependency

The CI prerequisite #14093 has merged. This PR depends on the terminology sequence #14105 → #14109 and should rebase after those documentation prerequisites merge. Unrelated flaky product-test checks are not part of this documentation PR's scope.

Why this contract is needed

• release/3.18 does not have a checked-in canonical OpenAPI contract for its Admin API.
• Reusing the newer 3.19 or master document would expose operations, schemas, defaults, or examples that do not belong to 3.18.
• The contract previously carried API7-specific metadata and links; Apache APISIX should own the canonical source used by downstream documentation.
• Offline parity validation is needed so future edits cannot silently drift from the branch's Lua resources and schemas.

Summary

• add a version-appropriate OpenAPI 3.1 contract for the 3.18 Admin API
• validate the operation surface, source schemas, references, examples, defaults, and release version entirely from checked-in files
• replace API7-specific metadata, links, and examples with version-pinned Apache references
• keep terminology corrections independently reviewable in docs: backport terminology corrections to 3.18 #14109

Validation

• one commit containing only the OpenAPI contract, validator, tests, and validator requirements
• offline source-parity validator passes
• 8 mutation and negative tests pass
• Redocly reports a valid OpenAPI document
• git diff --check origin/release/3.18...HEAD
• all Apache documentation targets return HTTP 200
• the specification contains no API7-specific literals

[kayx23]

CI note: the failing legacy jobs stop before this documentation change or its contract checker runs. The release-branch workflows now execute on Ubuntu 26.04, where actions/setup-python cannot provide Python 3.9 and apt no longer provides libpcre3/libpcre3-dev. The documentation, license, and PR-title checks pass. The version-specific offline OpenAPI source-parity checker and its regression tests also pass locally in a clean isolated environment; validation details are included in the PR description.