Fixes #1644.

Summary

dynamic_context(...) currently does not work with rig-surrealdb because the blanket
VectorStoreIndexDyn impl in rig-core assumes backends whose filter value type is
serde_json::Value, while SurrealSearchFilter intentionally uses native
surrealdb::types::Value.

This PR fixes that incompatibility without changing the public SurrealDB filter API.

What changed

• kept SurrealSearchFilter::Value = surrealdb::types::Value
• added TryFrom<Filter<serde_json::Value>> for SurrealSearchFilter
• added an explicit VectorStoreIndexDyn impl for SurrealVectorStore
  that converts canonical JSON filters with try_map_filter(SurrealSearchFilter::try_from)
  before delegating to the native VectorStoreIndex implementation
• used SurrealDB’s native Value::from_t(...) conversion path instead of a custom JSON-to-Value mapper
• added regression tests for:
  • nested JSON filter conversion
  • dyn compatibility for SurrealVectorStore
• added a clearer vector_store example with explicit top-hit assertions
• updated SurrealDB example targets to require rig-core/rustls, since they call OpenAI

Manually ran:

• cargo run -p rig-surrealdb --example vector_store --features rig-core/rustls

The updated example now returns pasta carbonara as the expected top hit and
asserts that the thresholded query narrows to that single result.