Docs: clarify Poetry dev install and running tests

[Pradeep18102003]

Fixes #1097

• Use poetry install -E dev for installing dev/test dependencies.
• Add quick verification for pytest-cases.
• Clarify skipping integration tests by default and note Google ADC requirements.
• Add small Windows/VS Code notes to reduce setup confusion.

[Copilot]

Pull request overview

This PR updates CONTRIBUTING.md to clarify the developer setup experience: it adds a Windows note about spaces in paths, swaps --extras dev for the -E dev shorthand, adds a troubleshooting tip for pytest-cases, documents how to skip integration tests, and adds a shell-environment verification step.

Changes:

• Replaced poetry install --extras dev with poetry install -E dev and added a pytest-cases verification snippet.
• Added --ignore=tests/integration to pytest invocations and a callout about Google ADC requirements.
• Added Windows path-with-spaces warning and a poetry shell environment confirmation tip.

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