fix(model-install): keep restored downloads restartable

[primorLee]

Summary

Fix restored installs that receive a non-resumable response before their first download-started signal.

• Share a newly-created multifile download's parts with the public install job before queue submission
• Keep pre-start resume_required state visible to the API and available to the install marker and restart path
• Add a regression covering enqueue, pre-start cancellation, public serialization, marker persistence, and restart

Related Issues / Discussions

Closes #9480

QA Instructions

uv run --frozen --extra test pytest tests/app/services/model_install/test_model_install.py::test_restart_failed_uses_parts_created_before_download_started -q
INVOKEAI_ALLOW_PRIVATE_DOWNLOAD_URLS=true uv run --frozen --extra test pytest tests/app/services/model_install/test_model_install.py -q
INVOKEAI_ALLOW_PRIVATE_DOWNLOAD_URLS=true ./.venv/bin/pytest tests/app/services/download/test_download_queue.py -q -k 'not download_refuses_non_public_source and not download_refuses_redirect_to_non_public_address and not rejected_redirect_closes_streamed_response and not download_refuses_multi_hop_redirect_to_non_public_address'
./.venv/bin/pytest tests/app/services/download/test_download_queue.py::test_download_refuses_non_public_source tests/app/services/download/test_download_queue.py::test_download_refuses_redirect_to_non_public_address tests/app/services/download/test_download_queue.py::test_rejected_redirect_closes_streamed_response tests/app/services/download/test_download_queue.py::test_download_refuses_multi_hop_redirect_to_non_public_address -q
./.venv/bin/ruff check invokeai/app/services/model_install/model_install_default.py tests/app/services/model_install/test_model_install.py
./.venv/bin/ruff format --check invokeai/app/services/model_install/model_install_default.py tests/app/services/model_install/test_model_install.py

The focused regression passes, the full model-install test file passes 36 tests, and the related download-queue file passes all 36 tests when split between its mocked private-address cases and its four default-policy SSRF refusal cases. The split is needed locally because the sandbox maps public test hostnames into the reserved 198.18.0.0/15 range.

Merge Plan

No special merge sequencing is required.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Redux migration is not applicable; no redux slice changed
• Documentation is not applicable; behavior and public interfaces are unchanged
• What's New copy is not applicable; this is not a release PR