🩹 ci: Fix Invalid Workflow Blocking Package Publishing

[lia-by-librechat]

What broke

The reusable validate.yml workflow became invalid when #569 added five duplicate provider base URL keys beside the existing vars.* || secrets.* values. This prevented CI and publish runs from creating any jobs, including the failed v3.9.7 publish run.

Fix

• Remove the five redundant secret-only keys while retaining variable-first, secret-fallback resolution.
• Add a standalone PR/main workflow that parses every workflow YAML with the existing Prettier dev dependency. Because it does not call validate.yml, it reports duplicate mapping keys even when the reusable validation workflow cannot start.
• Cover the real workflows and a duplicate-key fixture in a focused Node test.

Verification

• The original workflow fails strict YAML parsing on duplicate OPENAI_BASE_URL; all four updated workflows parse cleanly.
• node --test config/workflow-syntax.test.mjs: 2 passed.
• prettier --check on all touched files: passed.
• node --check config/workflow-syntax.test.mjs: passed.
• tsc --noEmit: passed.
• GitHub Workflow Syntax: passed on the final PR head; CI validation is running. The repository ESLint config excludes config/*.mjs, so it does not lint the new test.

Release recovery

npm still reports 3.9.6 as latest. Merging will trigger a new main push that can publish pending v3.9.7. The existing GitHub release job only creates releases for a version-named head commit; the follow-up fix commit will not automatically backfill the v3.9.7 GitHub release. No merge or publish was performed in this PR.