feat(vision): ModelRouterVisionProvider + multimodal ChatMessage — bump nullain-sdk to 0.7.0

[netty-linux]

Summary

• VisionProvider (nullain.ports.vision) gains its first adapter, ModelRouterVisionProvider: routes an image + prompt through an injected ModelRouter/LLMProvider as a plain multimodal chat completion (PLAN.md Fase 2, rescoped — no heavy OCR/CV package needed).
• ChatMessage.content (nullain.llm.types) now also accepts list[TextPart | ImagePart] (new ContentPart union). Existing str | None path is byte-for-byte unchanged in to_api_dict().
• VisionError added to the error hierarchy, mirroring SearchError.
• Version: nullain-sdk 0.6.0 → 0.7.0 (MINOR). Neither ChatMessage nor VisionProvider/ModelRouterVisionProvider/SearchProvider are in nullain/__init__.py::__all__ (not covered by the formal SemVer policy per docs/api-stability.md), but the change is additive/non-breaking regardless — exactly a MINOR case in practice.
• CHANGELOG.md's [Unreleased] block (search + vision work) moved to a dated [0.7.0] section.

Test plan

• make check (ruff + pyright strict + pytest) green — 726 passed, 7 skipped
• test_vision_provider_contract.py runs against ModelRouterVisionProvider via an offline fake LLMProvider
• Manual diff of nullain-sdk/src/nullain/__init__.py — zero lines changed

Notes

• Filed scripts/bump_version.py desalinhado do repositório #80 (bump_version.py assumes single synced version across packages — doesn't match independent per-package versioning already in practice) and release.yml publica os 3 pacotes juntos em qualquer tag v*, incompatível com versionamento independente #81 (release.yml publishes all 3 packages on any v* tag, incompatible with the same) as separate tech-debt issues; not fixed in this PR.
• Cancelled 5 stale workflow_dispatch runs stuck in waiting for 1-3+ days.

🤖 Generated with Claude Code