Restore legacy SDK aliases for 0.1.3

[NaoyaTakashima]

Summary

• restore GenFlux and GenFluxError as backwards-compatible aliases
• bump package metadata to 0.1.3 and document the no-breaking-change release
• add regression tests that keep old and preferred import names equivalent

Verification

• uv run ruff check src tests test_quickstart.py test_setup.py
• uv run pytest tests/test_backward_compatibility.py -q
• uv lock --check
• uv run pyright src/
• uv run python -m build

Note: broader API integration-style tests that contact localhost:9000 were not used as a release gate because the platform backend was not running locally.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/sdk-compat-alias-0.1.3

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