fix: speed up existing devcontainer entry lookup

[ethanmillerinvestments-code]

/claim #24

Summary

• Optimized the existing devcontainer database lookup path by returning cached records before fetching GitHub repository context or invoking generation.
• Added focused tests covering cached hits, missing records, and regenerate behavior.
• Kept cache miss and regenerate behavior unchanged while avoiding redundant work for database hits.

Verification

• python3 -m unittest tests/test_devcontainer_lookup.py
• python3 -m unittest discover -s tests
• python3 -m compileall main.py helpers tests
• Notes: the cached-hit test proves one database lookup with zero repository fetches and zero generation calls. Full default python3 -m unittest discover is blocked locally by the repo's existing top-level test.py importing requests when dependencies are not installed.

Scope

Small targeted change for issue #24. No payout, auth, secrets, or private data changes.

Repository note

The original daytonaio/devcontainer-generator repo URL currently returns 404 through live GitHub API and git. This PR targets the public devcontainer-generator repository GitHub currently exposes for the codebase.