add bge-m3 embedding dimension alias

[TurboTheTurtle]

Summary

Allow users to configure Huawei/BAAI BGE-M3 with the plain bge-m3 model name without also having to set embedding.dimensions manually.

• Add bge-m3 as a 1024-dimension alias alongside the existing BAAI/bge-m3 entry.
• Extend the embedder regression test so both common model identifiers resolve to 1024 dimensions.

Fixes #339

Validation

• node test/embedder-error-hints.test.mjs
• git diff --check

[rwmjhb]

Please rebase this branch on current master and rerun CI before merge.

The bge-m3 alias change is small and useful, but this branch is behind current master and currently reports packaging/core failures. A fresh rebase should keep this as a minimal embedder/test-only diff.