fix(sqlite): deserialize JSON columns

[gold-silver-copper]

Summary

Fixes #1794.

• Decode declared SQLite JSON columns into structured JSON values when rebuilding documents from top_n results.
• Keep ordinary TEXT columns unchanged, including JSON-looking strings.
• Return a SQLite conversion error for invalid JSON stored in a declared JSON column.
• Add unit coverage for JSON/TEXT column conversion and a live sqlite-vector regression matching the issue's structured metadata field.

Root Cause

rig-sqlite rebuilt document rows as serde_json::Value::Object before deserializing them into the caller's document type. SQLite text values were always converted to serde_json::Value::String, even for columns declared as JSON. That made a stored metadata object like {"user_id":1,...} deserialize as a string instead of the expected Rust metadata struct.

Validation

• cargo fmt --check
• cargo test -p rig-sqlite json_column
• cargo test -p rig-sqlite text_column_json_looking_text_stays_string
• cargo test -p rig-sqlite
• cargo clippy -p rig-sqlite --all-targets --all-features
• git diff --check