fix: protect URL downloads against SSRF with pinned DNS

[banned2054]

Fixes #10122

URL-based file and image downloads could reach private services through a direct URL, a DNS response, or a redirect. This change validates each destination before requesting it and pins the validated DNS results to the connection.

Modifications / 改动点

• Protect download_file and download_image_by_url in the shared download layer: accept HTTP(S) URLs without embedded credentials and reject non-public IPv4/IPv6 destinations by default.

• Use yarl hostname normalization, a resolver with no unchecked fallback, and explicit redirect validation. Apply the same policy to GET, POST, and TLS fallback requests.

• Preserve access to explicitly configured self-hosted T2I endpoints. For Telegram voice/audio downloads, permit custom private file services while restricting the initial request and every redirect to the configured scheme, host, and port.

• Document the proxy compatibility change in both English and Chinese configuration guides.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Compatibility: Protected downloads now connect directly with trust_env=False. They no longer use HTTP(S) environment proxies, because proxy-side DNS resolution would bypass the pinned destination. Deployments requiring a proxy need a directly reachable download URL or an appropriate application-level mirror. Existing callers downloading private URLs must explicitly opt into trusted private destinations.

Screenshots or Test Results / 运行截图或测试结果

Verification on Windows using the project virtual environment:

python -m pytest tests/unit/test_io_download_file.py tests/test_telegram_adapter.py -q -p no:cacheprovider
47 passed, 1 warning

ruff check .
All checks passed!

ruff format --check .
513 files already formatted

git diff --check
(no errors)

The warning is the existing audioop deprecation warning. Tests cover blocked IP/DNS destinations, IDNA normalization, redirect validation, trusted-origin restrictions, proxy settings, POST handling, TLS fallback behavior, and Telegram integration. A real aiohttp connector test uses a mocked hostname and a local server with a same-origin redirect, asserting one DNS lookup per validation step and no additional lookup when either connection is opened.

Checklist / 检查清单

• New features have been discussed with the authors. Not applicable: this is a bug fix for [Bug] 通过URL发送文件功能缺少SSRF防护，存在内网探测风险 #10122.
• Verification steps and test results are provided above.
• Documentation impact reviewed: English and Chinese proxy notes updated; no WebUI navigation, structure, or label changes requiring screenshots or an entry mapping.
• No new dependencies introduced.
• No malicious code introduced.

Summary by Sourcery

Protect outbound media downloads against SSRF while preserving controlled access for trusted self-hosted services.

New Features:

• Add SSRF-safe URL downloads with DNS validation, pinned connection addresses, redirect checks, and support for GET, POST, and TLS fallback requests.
• Allow explicitly configured self-hosted T2I endpoints and custom Telegram file services to access private origins under origin restrictions.

Bug Fixes:

• Prevent file and image downloads from reaching private or otherwise unsafe destinations through direct URLs, DNS responses, or redirects.
• Ensure locally materialized images are used instead of private file-service URLs when converting or rendering media.

Enhancements:

• Prefer local media paths across image conversion and Satori rendering, while preserving configured file-service URLs for distribution.
• Disable environment HTTP(S) proxy use for protected downloads and document the resulting deployment requirements.

Documentation:

• Document the proxy compatibility implications of SSRF-protected downloads in English and Chinese configuration guides.

Tests:

• Add coverage for blocked private addresses and DNS results, IDNA normalization, origin and redirect restrictions, proxy behavior, POST requests, TLS fallback, pinned DNS connections, Telegram integrations, and self-hosted T2I rendering.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Needs a human reviewer. This changes the outbound-request trust boundary for all protected downloads: a validation or redirect/DNS-pinning mistake could let attacker-controlled URLs reach private services and feed their responses into the application. Reverting would stop future requests but would not undo any internal data already fetched or processed.

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[silvaling]

🔴 [blocking] 自建私网 T2I 端点在 return_url 路径下会被新策略拦掉

本 PR 为私网自建渲染端点留的 allow_private_network 豁免，只覆盖了 return_url=False 分支；本体默认走的 return_url=True 路径没有被覆盖。

链路（master 行号）

1. pipeline/result_decorate/stage.py:382 → render_t2i(..., return_url=True)，:400 变成 Image.fromURL(url)
2. utils/t2i/network_strategy.py:189 → f"{endpoint}/{id}"，endpoint 即 t2i_endpoint，可为私网
3. 发送侧取本地文件：QQ aiocqhttp_message_event.py:38-40 走 convert_to_base64()；Telegram tg_event.py:312、WebChat message_parts_helper.py:418、Lark/Slack/企微 走 convert_to_file_path()
4. 两条都落到 utils/media_utils.py:486 的 download_file(...)，此处 allow_private_network 默认 False
5. 本 PR 的豁免加在 network_strategy.py:193-203，即另一条分支

影响：t2i_endpoint 指向私网自建的部署，渲染成功但图片随后被拦、发不出图；官方端点与公网端点不受影响。t2i_use_file_service=True 也无法规避（message/components.py:575 同样要下载）。

备注：静态链路追踪，未实机复现。

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