#360 fix: correct SEA binary setup instructions

Summary

• point missing-binary errors to the existing scripts/download_binary.py helper
• replace the stale README anchor with the local-development documentation in CONTRIBUTING.md
• add a regression assertion for both actionable references

Testing

• uv run --frozen pytest tests/test_sea_binary.py -q (5 passed)
• uv run --frozen ruff check .
• uv run --frozen pyright -p .
• uv run --frozen mypy --platform linux .
• full Python 3.9 and 3.14 suites run on Windows; only the pre-existing Windows failures addressed by #355 remained

Fixes #353

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Summary by cubic

Corrects SEA missing-binary error guidance so local development instructions are accurate. Previously the error pointed to a non-existent script and a stale README anchor; it now references the correct helper script and CONTRIBUTING.md. No behavior change beyond error text.

• Error now suggests: "uv run python scripts/download_binary.py" (replaces "scripts/download-binary.py").
• Updates the help link to https://github.com/browserbase/stagehand-python/blob/main/CONTRIBUTING.md.
• Adds a regression test asserting both the command and link in the error message.

Fixes #353.