Cache per-class field alias info in UniversalBaseModel

[rsd-darshan]

Fixes #802

_coerce_field_names_to_aliases is a mode="before" validator on UniversalBaseModel, so it runs on every model instantiation, including every nested model parsed out of an SSE event during chat_stream. It was rebuilding the field name/alias maps from scratch on each call even though they're constant per class.

This caches the per-class alias info (name→alias map, the subset of names whose alias differs, and the ambiguous-key set) keyed by class id, mirroring the existing _type_adapter_cache. Two fast paths are added on top:

• If a class has no differing aliases, the input is returned unchanged.
• If none of the incoming keys need rewriting, the input is returned unchanged, skipping the dict copy.

parse_obj_as's own has_pydantic_aliases scan is also switched to reuse this cache instead of walking the fields a second time.

Testing

• Verified output is identical to the pre-change validator across aliased fields, plain names, non-aliased models, and the existing ambiguous-key path.
• ruff check and mypy pass on the changed file.
• pytest run locally: all failures are pre-existing/unrelated (live API tests requiring a real key, and one unrelated mypy-stub gap in oci_client.py).

---

Note

Low Risk
Refactor of existing alias coercion logic with intended behavioral parity; main risk is a subtle edge case in fast-path or ambiguous-key handling, not security or data integrity.

Overview
Speeds up UniversalBaseModel validation (notably nested models in SSE/chat_stream) by computing field name→alias maps once per class instead of on every instantiation.

Introduces _get_field_alias_info with a class-id cache (same pattern as _type_adapter_cache) holding name_to_alias, fields whose alias differs, ambiguous keys, and keys that might need rewriting. parse_obj_as reuses differing_names from that cache instead of scanning model fields for real Pydantic aliases.

_coerce_field_names_to_aliases (Pydantic v1 and v2) now pulls from the cache and adds two early exits: return input unchanged when the class has no alias/name mismatches to care about, or when incoming keys do not intersect relevant_keys; only then copy the dict and rewrite keys in differing_names. Ambiguous-key errors are unchanged.

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