Remove initial_metadata from internal metadata construct

[sydney-runkle]

• Remove initial_metadata parameter from build_metadata_dict that encouraged abuse of the metadata construct by magic string injection
• Improve docstring of CoreMetadataHandler class with TODO relating to next steps for a refactor (I'm happy to investigate this for v2.10)

I investigated removing the metadata property from CoreMetadataHandler due to the abundance of shared logic with __init__ in the same class, but it appears that this is necessary due to the mutation of metadata on the schema passed in. I'll note, in terms of best practices, this should certainly be changed with the aforementioned refactor - the less in place mutation we can do for core schemas, the better.

[sydney-runkle]

I'll note, the docstring for CoreMetadata could most certainly be improved, but it's honestly hard to tell exactly what's going on with the first two fields (specifically, the differences between the two). I might defer this until the refactor when we can hopefully remove these :)

[codspeed-hq]

CodSpeed Performance Report

Merging #10194 will not alter performance

_{Comparing simplify-metadata (e6507a8) with main (8cd7557)}

Summary

✅ 17 untouched benchmarks

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_core_metadata.py
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