fix: mock embeddings module to prevent onnxruntime-web worker error in tests (#20)

Github Action failure:
https://github.com/reddit/devvit-mcp/actions/runs/19406523269/job/55521967266

The @xenova/transformers library uses onnxruntime-web which attempts to
create web workers using blob URLs. This fails in Node.js test
environments
with the error: 'The worker script or module filename must be an
absolute
path or a relative path starting with './' or '../'. Received blob:...'

By mocking the embeddings module in DocumentManagementService.test.ts,
we prevent the transformers pipeline from initializing during tests,
avoiding the worker script error while maintaining test coverage.

Author: sp3nx0r