Add RemoteContext Python bindings for HTTP client

[beinan]

Summary

• Expose lance-context-client's RemoteContextStore to Python via PyO3, enabling Python code to talk to a remote lance-context-server over HTTP
• Add async RemoteContext wrapper class mirroring the AsyncContext pattern
• Fix pre-existing test mock signature mismatch in test_embeddings.py

Changes

• python/Cargo.toml: Add lance-context-api and lance-context-client path dependencies
• python/src/lib.rs: Implement RemoteContext pyclass with all ContextStoreApi methods
• python/python/lance_context/api.py: Add async RemoteContext wrapper class
• python/python/lance_context/__init__.py: Export RemoteContext
• python/tests/test_embeddings.py: Fix _DummyInner.add() signature to include state_metadata

Usage

import asyncio
from lance_context import RemoteContext

async def main():
    ctx = await RemoteContext.connect("http://localhost:3000", "my-context")
    await ctx.add(role="user", content="hello world")
    records = await ctx.list()
    print(records)

asyncio.run(main())

Test plan

• maturin develop --release builds successfully
• RemoteContext is exported from lance_context package
• Existing tests pass (143/148 pass; 2 S3 tests skip due to missing bucket, 2 skipped, 1 xfail)
• Smoke test against running lance-context-server

🤖 Generated with Claude Code