fix(ssrf): guard explicit proxy connections

[wunianze666-netizen]

Summary

• install InvokeAI's guarded HTTP/HTTPS connection pools on Requests proxy managers
• prevent an explicit download_proxy from bypassing the connected-peer SSRF check
• add a regression test using a loopback proxy

Related Issues / Discussions

Refs #9493 (download-queue SSRF follow-up, item 2).

QA Instructions

• Verified the pre-fix behavior locally: an explicit loopback proxy returned HTTP 200 despite the guarded session.
• Verified the new behavior locally: the same request is rejected with UnsafeDownloadURLException.
• python -m ruff check invokeai/app/util/ssrf.py tests/app/util/test_ssrf.py
• python -m ruff format --check invokeai/app/util/ssrf.py tests/app/util/test_ssrf.py
• Python compilation and git diff --check pass.

Merge Plan

This is intentionally limited to the proxy-manager path described in #9493. It does not change the explicit private-download opt-out or the separate injected-session follow-up.

Checklist

• I have performed a self-review of my own code
• I have added or updated focused tests
• I have checked the diff for secrets and unrelated files

[wunianze666-netizen]

Synced this branch with current main (ef832d1aa) in merge commit 1b66e1299. The PR diff remains limited to ssrf.py and its focused test. Revalidation on the updated tree: the explicit-loopback-proxy regression passed; 68 additional SSRF tests passed; Ruff check/format, Python compilation, and git diff --check passed. One unrelated host-specific test (test_no_warning_without_a_proxy) sees the Windows system proxy registry and fails even after proxy environment variables are cleared; I reproduced the identical failure on a clean detached origin/main, so it is not introduced by this PR. Fresh full CI is now running.

[wunianze666-netizen]

Added HTTPS CONNECT-tunnel coverage in e92a43935 so the regression now proves the connected-peer guard rejects a loopback explicit proxy for both plain HTTP requests and HTTPS tunnels. Focused proxy cases: 2/2 passed. The broader SSRF file passed 69 tests with only the previously documented host-dependent system-proxy case excluded; Ruff check/format and git diff --check passed.