Python(feat): expose multi-value metadata via a Metadata mapping with getall

[dengjonathan]

What

Stacked on #698 (crash fix). That PR makes multi-value metadata safe to fetch; this one makes it usable: values beyond the first are no longer invisible to client code.

metadata_proto_to_dict now returns Metadata, a dict subclass:

run = client.runs.get(run_id=...)
run.metadata["associated_parts"]           # "ABC" — first value; all existing dict behavior unchanged
run.metadata.getall("associated_parts")    # ["ABC", "XYZ"] — full list, canonical order
run.metadata.getall("missing")             # []

• Scalar view = first value per key, so every existing caller (dict access, .get(), iteration, ==, isinstance(md, dict), json.dumps) behaves exactly as before. Invariant: md[k] == md.getall(k)[0].
• Run / Asset / Report / Channel .metadata fields adopt the type. A __get_pydantic_core_schema__ hook passes Metadata instances through validation untouched — without it, pydantic rebuilds the field as a plain dict and silently drops the extra values. Serialization (model_dump / model_dump_json) still emits a plain first-value dict, so dumped output is unchanged.
• Follows the MultiDict pattern (werkzeug getlist, aiohttp getall, stdlib email.get_all); getall matches aiohttp's spelling. Same contract as the rule-payload Metadata shipping in azimuth (ENG-13104), so rule code, canvas code, and client scripts share one metadata API.

Scope

Read side only. The write path (accepting list values in metadata_dict_to_proto / update models) is Phase 3 of ENG-13281.

Testing

_tests/util/test_metadata.py grows from 4 to 13 tests: getall ordering/copy/fallback semantics, first-value invariant, JSON serializability, and pydantic field behavior (instance pass-through preserving lists, plain-dict wrapping, non-mapping rejection, model_dump output). Also verified end-to-end that Run._from_proto with duplicate-key metadata yields metadata.getall(...) with the full list. ruff check / ruff format --check pass at the pinned version (0.12.12); generated sync stubs don't reference the changed annotations.

Linear: ENG-13281 (Phase 2)

🤖 Generated with Claude Code

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-699/

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