fix(external): guard AlibabaCloud image downloads

[wunianze666-netizen]

Summary

• route AlibabaCloud image URLs returned by DashScope through InvokeAI's socket-level SSRF-guarded session instead of the module-level requests.get() path
• reject loopback/private peers at the connected socket, preserving protection across redirects and DNS rebinding
• retain the existing streamed 32 MiB response cap and translate refused URLs into the provider error boundary
• add a regression that fails if provider-response downloads fall back to unguarded requests

The submit and task-poll requests are unchanged; this patch only hardens the follow-up fetch of provider-generated image URLs.

Related Issues / Discussions

Refs #9493, specifically follow-up item 4.

This PR intentionally does not close #9493 because that issue tracks several independent security follow-ups.

QA Instructions

• confirmed the new regression fails on unmodified main because _download_image() calls unguarded requests.get()
• python -m pytest tests/app/services/external_generation/test_alibabacloud_provider.py -q — 9 passed
• python -m pytest tests/app/services/external_generation -q — 46 passed
• focused socket-guard coverage for loopback, DNS rebinding, percent-encoded hosts, and both URL schemes — 4 passed
• Ruff check and format-check passed for both changed files
• git diff --check passed

Reviewer focus: build_guarded_session() intentionally ignores ambient proxy variables so destination resolution remains in-process and the connected peer can be checked. This matches the existing download security boundary.

Merge Plan

No special merge steps are required. The patch is limited to the AlibabaCloud provider and its focused tests; it changes no schemas, generated assets, configuration, or release metadata.

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 state changed
• Documentation is not applicable; no public API or configuration changed
• What's New copy is not applicable; this is not a release PR