librarian: refactor helper functions in add/generate to a dedicated package

The add/generation logic is similar among languages, e.g., gather protos from API path.

We should identify common logic and refactor to a dedicated package.

Start by tracing the add/generation logic across the language implementations, especially the repeated work that gathers protos from an API path. Identify the shared behavior and its current entry points before deciding what belongs in a dedicated package. Done means the common logic is centralized without changing generation behavior.

Written by the indexing model from the issue text.