feat: pluggable embedding provider registry

[dcfocus]

Summary

• Adds an EmbeddingProvider protocol and registry so Context auto-embeds text at write time and auto-embeds string queries at search time — eliminating the per-caller embedding glue described in Proposal: pluggable embedding provider registry (OpenAI / Jina / local) for auto-embedding on ingest #85
• Ships two built-in providers (OpenAIProvider, SentenceTransformersProvider) behind optional extras; accepts any object satisfying the protocol for custom backends
• Manual embedding= always takes precedence; existing callers are unaffected

API

# built-in provider via dict shorthand
ctx = Context.create(
    "context.lance",
    embedding={"provider": "openai", "model": "text-embedding-3-small"},
)

# or pass a provider instance (including custom)
from lance_context import EmbeddingProvider

class MyProvider:
    @property
    def dims(self) -> int: return 768
    def embed_texts(self, texts: list[str]) -> list[list[float]]: ...

ctx = Context.create("context.lance", embedding_provider=MyProvider())

ctx.add("user", "Where should I travel in spring?")  # auto-embedded
results = ctx.search("spring travel")                # query auto-embedded

Changes

File	What
lance_context/embeddings.py	EmbeddingProvider protocol, OpenAIProvider, SentenceTransformersProvider, _build_provider registry
lance_context/api.py	embedding_provider kwarg on __init__/create; auto-embed in add, upsert, add_many, search; provider propagated through fork
lance_context/__init__.py	exports EmbeddingProvider
pyproject.toml	[openai] and [sentence-transformers] optional extras
tests/test_embeddings.py	15 new tests (stub provider, no external deps)

Notes

• retrieve() is intentionally not wired: its text arg feeds BM25 (not the vector index), so a separate vector= kwarg is already the right surface if hybrid retrieval with auto-embedding is wanted in a follow-up
• add_many() sends all un-embedded text records in one provider call — batching is preserved end-to-end

Test plan

• python/tests/test_embeddings.py — 15 unit tests, zero external deps
• Existing test_search.py, test_add_many.py, test_delete.py, test_async.py — all pass (85 total across these suites)

Closes #85

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