fix: emit missing symbol information

[brendan-m-murphy]

Fixes #223.

Ensures cached class definitions still emit SymbolInformation and records identity-only SymbolInformation for undocumented standard-library symbols.

Adds the reported reproduction and structural regression checks.

[brendan-m-murphy]

I know it's bad form... but this is one-shot from codex. This PR does fix the bug I encountered though:

I tested this fix against OpenGHG, a medium-sized Python package for processing atmospheric greenhouse-gas measurement and modelling data.

Using the same OpenGHG checkout and scip-cli 2.7.0:

• Released scip-python 0.6.6 indexed 275 files, but conversion to SQLite failed because VersioneerBadRootError# had a definition occurrence without corresponding SymbolInformation.
• A build from this PR also indexed 275 files and converted successfully.
• The resulting 7.2 MB database contained 6,666 global symbols and 18,650 mentions.
• As a functional check, scip-cli successfully resolved Datasource.add_timed_data to its definition in openghg.objectstore._legacy_datasource.

This is a controlled before/after test and confirms that the PR fixes the missing-SymbolInformation failure on a real-world Python codebase.

(Again, apologies for the AI generated summary... scip-python seems useful and I want to try it out, but I don't have time to dig into the code myself.)