fix: correct SEA binary setup instructions

[abhinavkr26104]

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 Make byte fixtures checkout-stable and run tests on Windows CI #355 remained

Fixes #353

---

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.

^{Written for commit a25a20e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant Dev as Developer
    participant App as Stagehand App
    participant Resolver as resolve_binary_path()
    participant Env as Environment
    participant FS as File System
    participant Docs as Documentation

    Note over Dev, Docs: SEA Binary Resolution Flow with Corrected Guidance

    Dev->>App: Run stagehand code
    
    App->>Resolver: Call resolve_binary_path()
    
    Resolver->>Env: Check STAGEHAND_SEA_BINARY
    alt Environment variable set
        Env-->>Resolver: Custom binary path
        Resolver->>FS: Use custom binary
        FS-->>Resolver: Binary exists
        Resolver-->>App: Return binary path
    else No environment variable
        Env-->>Resolver: Not set
        Resolver->>FS: Look for default SEA binary
        FS-->>Resolver: File not found
        Note over Resolver, Docs: Error path with corrected instructions
        Resolver->>Resolver: Construct FileNotFoundError
        Resolver-->>Dev: Raise error with guidance
        Note over Dev, Docs: Error message contains corrected details
        Dev->>Docs: Read error message
        Docs-->>Dev: Instructions to run download script
        Dev->>Dev: Run "uv run python scripts/download_binary.py"
        Dev->>FS: Download SEA binary
        FS-->>Dev: Binary available
        Dev->>App: Retry stagehand code
    end

    Note over Dev, Resolver: Regression test validates error message content
    Dev->>Resolver: Test with missing binary
    Resolver-->>Dev: Verify correct script name and CONTRIBUTING.md link

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_{Re-trigger cubic}