chore: introduce tests extras for lighter install (FXC-4160)

[daquinteroflex]

Required because full dev installation is consuming too much memory of the runners and sometimes they error

Greptile Summary

• Adds tests extras group to pyproject.toml with 11 test-related dependencies
• Enables lighter installations for test environments without requiring all dev dependencies

Confidence Score: 5/5

• This PR is safe to merge with no risk
• Simple, well-structured dependency reorganization with no code changes, only configuration updates that follow poetry's standard practices
• No files require special attention

Important Files Changed

Filename	Overview
pyproject.toml	Introduces new tests extras group with 11 testing-related dependencies to enable lighter installations without full dev dependencies

Sequence Diagram

sequenceDiagram
    participant User
    participant pip
    participant pyproject.toml
    participant poetry
    
    User->>pip: pip install tidy3d[tests]
    pip->>pyproject.toml: Read extras configuration
    pyproject.toml-->>pip: Return tests dependencies list
    pip->>poetry: Resolve dependency markers
    poetry-->>pip: Provide resolved packages
    pip->>User: Install lightweight test environment

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