feat: add preflight validation

[Achal13jain]

Summary

Adds a new vectormeta validate preflight command and reusable validate_records() API for checking vector records before upsert.

This validates metadata size, missing or duplicate IDs, vector shape, vector dimension consistency, optional --dim matching, and Pinecone metadata value rules. It also adds structured validation reports with error/warning severity, table and JSON output, tests, and documentation updates.

Testing

python -m pytest
ruff check .
ruff format --check .
mypy vectormeta

Checklist

• Core logic remains independent from Typer/Rich where practical.
• New behavior has focused tests.
• Docs or examples are updated when user-facing behavior changes.
• Vector database limit claims are cautious and sourced where needed.