Add testcontainers for integration testing

[nazq]

Summary

Adds testcontainers support for running integration tests without manual Docker setup. Tests automatically spin up a Qdrant container and clean up after completion.

Changes

New Files

• tests/test_utils.rs - Qdrant container setup with tmpfs for fast in-memory testing
• tests/integration.rs - Integration tests using containerized Qdrant

Modified Files

• Cargo.toml - Added testcontainers 0.26 dev-dependency
• src/lib.rs - Marked legacy integration test as #[ignore]
• src/qdrant_client/collection.rs - Marked legacy integration test as #[ignore]
• src/qdrant_client/query.rs - Marked legacy integration test as #[ignore]

Features

• Shared container instance across tests for efficiency
• tmpfs mounts for zero disk I/O during tests
• Configurable Qdrant version via QDRANT_VERSION env var
• HTTP health check wait strategy

Usage

# Run integration tests (container starts automatically)
cargo test --test integration

# Run with specific Qdrant version
QDRANT_VERSION=v1.15.0 cargo test --test integration

Test Plan

• Integration tests pass with containerized Qdrant
• Existing unit tests unaffected
• Legacy integration tests properly ignored