feat(seedream): harden marketplace readiness

[acedatacloud-dev]

Why

The Seedream plugin needed an auditable reviewer flow, accurate data handling boundaries, safe package exclusions, and fail-closed credential/error behavior before any marketplace handoff.

What

• document all tools, fixed endpoint, billable reviewer check, support, and async limitation
• replace generic privacy text with the actual plugin data flow and retention boundary
• exclude tests and .env.* files from plugin packages
• reject empty/bare Bearer credentials and redact the configured token from returned errors
• add marketplace-readiness and security regression tests

Evidence

• PYTHONPATH=$PWD/plugins/seedream python3 -m pytest -q plugins/seedream/tests (11 passed)
• ruff check and ruff format --check on changed Python files
• git diff --check
• staged secret/internal-sourcing scan passed

External gate

manifest.yaml intentionally remains verified: false. This PR does not submit to or claim approval/publication by Dify Marketplace. A funded reviewer request remains an owner-authorized external step.

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