chore: enforce future annotations in src

[chethanuk]

📋 Summary

AGENTS.md and STYLEGUIDE.md require from __future__ import annotations in every source file, but nothing enforces it, and 16 __init__.py files under packages/*/src don't have it. This adds ruff's required-imports (I002) for packages/*/src and adds the import to those files.

🔗 Related Issue

Closes #760

🔄 Changes

• Uses I002 instead of the FA102 the issue suggests. FA102 only fires for targets below py310, so with target-version = "py310" it passes on main even with these files missing the import.
• "!packages/*/src/**" = ["I002"] in per-file-ignores keeps tests, tests_e2e and scripts out of scope.
• I002 skips files with no statements, so 14 of the 16 files (header-only stubs) get the import but aren't guarded until they have code.

🧪 Testing

• make test passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)
• make lint passes. Deleting the import from engine/storage/__init__.py gives I002 Missing required import; an existing tests file without it (config/analysis/conftest.py) is not flagged.
• The touched packages import cleanly, and pytest packages/data-designer-engine/tests/engine/storage packages/data-designer/tests/integrations passes (158 tests).

✅ Checklist

• Follows commit message conventions
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[github-actions]

Linked Issue Check

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge. No concrete bug was found.

Summary

This PR makes Ruff enforce from __future__ import annotations across package source files. It also adds the import to 16 affected __init__.py files while keeping tests, scripts, and end-to-end checks outside this rule.

• Adds Ruff's I002 required-import check for packages/*/src.
• Excludes tests, scripts, and end-to-end files from that check.
• Updates the affected package initializers without changing their existing exports.

Diagram

sequenceDiagram
    actor Dev
    participant Ruff
    participant Config as pyproject.toml
    actor User
    participant Python
    participant Package

    Dev->>Ruff: Run ruff check
    Ruff->>Config: Load lint rules
    alt "File is under packages/*/src"
        Config-->>Ruff: Require future annotations import
        alt File has statements and misses the import
            Ruff-->>Dev: Report I002
        else Import exists or file has no statements
            Ruff-->>Dev: Pass I002
        end
    else File is outside package src
        Config-->>Ruff: Ignore I002
    end

    User->>Python: Import a changed package
    Python->>Package: Run __init__.py
    Package->>Python: Enable postponed annotations for this module
    Package-->>User: Keep existing exports

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[github-actions]

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