Support redis-py 8

[abrookins]

Summary

• allow redis-py 8 and add it to the version test matrix
• keep RedisVL-created clients on RESP2 by default for backward compatibility
• support redis-py 8 RESP3 index metadata and pipelined search replies for user-provided clients
• align storage and embedding-cache types with redis-py 8
• make the MCP spoofed-host test send the requested wire-level Host header

Validation

• uv sync --all-extras --frozen
• redis-py 8.1.0 installed
• full suite: 2077 passed, 143 skipped, 2 xfailed
• focused redis-py 8 and connection tests: 82 passed, then 2 vector RESP3 integration tests and 10 protocol tests passed after final coverage changes
• make check-format
• make check-sort-imports
• make check-types
• uv run pre-commit run --all-files
• git diff --check

make check-lint still reports the repository's existing Pylint findings. CI does not run that target.

Closes #686