fix: Search limit validation

[Rudra-G-23]

Summary

Closes #22

• Validate limit and candidates before database access in search().
• Raise ValueError when either value is non-positive.
• Add regression tests covering zero and negative values.
• Ensure invalid values are rejected before opening a database cursor.

Testing

• Full test suite: 94 passed
• Issue search() passes limit/candidates into SQL with no guard #22 regression tests: passed
• PostgreSQL + pgvector integration: passed
• Ruff checks on changed files: passed
• Python compilation: passed
• git diff --check: passed

[github-actions]

Welcome, @Rudra-G-23, and thanks for your first pull request to VaultRAG.

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I'll review this shortly. Ask anything in the thread if something is unclear.

[github-actions]

Merged. Thanks a lot, @Rudra-G-23.

Good, focused change. Exactly the kind of thing VaultRAG needs.

If VaultRAG is useful to you, a star goes a long way.

Happy to see more from you whenever you have the time.