docs: Add TESTING.md and DEVELOPMENT_SETUP.md guides

[db-tycoon-stephen]

Problem

No contributor guides for testing or local development setup. New developers don't know how to write tests or get started.

Proposed Solution

TESTING.md

• How to run tests (uv run pytest)
• Test structure and conventions
• How to write new tests (patterns, fixtures, mocking Snowflake)
• Integration vs unit test guidelines
• Coverage reporting

DEVELOPMENT_SETUP.md

• Prerequisites (Python, UV, openssl)
• Clone and install steps
• Environment configuration (.env setup)
• IDE configuration (VS Code recommended settings)
• Pre-commit hook setup
• Common development workflows

Acceptance Criteria

• docs/contributing/TESTING.md created
• docs/contributing/DEVELOPMENT_SETUP.md created
• Linked from CONTRIBUTING.md and README