fix(preview): accept deprecated field= kwarg on PreviewTextQuery

[jhamon]

Summary

The rename PreviewTextQuery.field (str) → PreviewTextQuery.fields (list[str]) at fefc4850 is a breaking call-site change for any user written against pre-rename PreviewTextQuery. The preview namespace's documented SemVer policy permits this, but the cost is small to make it non-breaking, so this PR does.

• field= is reintroduced as a deprecated kwarg/attribute on the canonical msgspec.Struct (no parallel class, per the shim-purity convention).
• When provided, it migrates to fields=[field] in __post_init__ and emits a DeprecationWarning.
• omit_defaults=True keeps the encoded payload free of a "field": null artifact; existing wire-shape tests are unchanged.
• Decoding the legacy single-field backend response variant ({"type": "text", "field": "...", "query": "..."}) now works and routes through the same migration path.
• Mutually exclusive: passing both field= and fields= raises ValueError; passing neither raises ValueError.

Why

Avoids breaking callers without forcing a re-design of the preview API. The backend already accepts both wire forms, so this only restores the Python-side call signature.

Test plan

• uv run pytest tests/unit/preview/models/test_score_by.py — 21/21 pass, including 6 new tests covering the deprecation path
• uv run pytest tests/unit/preview/ — 705 passed, 1 xfailed (no regressions)
• uv run mypy --strict pinecone/ — clean across 186 source files
• uv run ruff check + ruff format --check — clean
• Integration tests in tests/integration/preview/ — out of scope for this PR (no behavior change on the wire)

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Note

Low Risk
Low risk: changes are confined to preview model initialization/serialization with added validation and tests; main risk is tightening constructor requirements for callers who passed neither fields nor field.

Overview
Restores backward compatibility for PreviewTextQuery by reintroducing deprecated field support that auto-migrates to fields=[...] and emits a DeprecationWarning, while enforcing that callers provide exactly one of field or fields.

Ensures JSON encoding stays canonical (no "field": null) via omit_defaults=True, and adds unit coverage for migration, validation errors, and decoding the legacy backend {"field": ...} variant.

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