test: add multi-step transaction sequence test

[TheShnider]

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🔗 Related Issues

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Closes #532

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🧪 Changes Made

• Bug fix
• New feature
• Refactor
• Documentation update

✅ Checklist

• Code compiles successfully
• Tests added/updated and passing
• Linting passes (no warnings/errors)
• Documentation updated (if required)
• No breaking changes (or clearly documented)

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[davidmaronio]

this is titled/issue-linked as "add a test" but the diff does a lot more, and a couple of things block it:

1. it commits a stray helper script fix_tests.py to the repo root (regex text-munging, build cruft). remove it.
2. the actual new test in src/tests/test_multi_step.rs only does create_campaign then cancel_campaign. that is not a multi-step transaction sequence (no contribute/withdraw/refund), so it does not exercise what the title implies. please make it a real sequence.
3. it rewrites types.rs (new MaybeAddress enum, CreateCampaignParams.token changed from Option), admin.rs verify_milestone, campaigns/create.rs etc. none of that belongs in a test PR and it collides with #575/#578. drop the unrelated changes.