fix: upload loopback image URLs as Discord attachments

[wcqqq1214]

Fixes #9756.

text_to_image() defaults to loopback URLs such as http://127.0.0.1:<port>/.... Discord's _parse_to_discord treated any http* image as a public URL and put it in discord.Embed.set_image(). Discord's servers cannot fetch loopback hosts, so the image rendered as a blank bar.

Loopback hosts (127.0.0.0/8, localhost, ::1, IPv4-mapped ::ffff:127.0.0.1) now fall through to the existing MediaResolver path and are uploaded as discord.File attachments. Public HTTP(S) URLs are unchanged.

Modifications / 改动点

• astrbot/core/platform/sources/discord/discord_platform_event.py: parse the image URL host and skip the Embed path when ipaddress.ip_address(...).is_loopback (or localhost) is true.

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

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

Before: Discord rendered the local T2I URL as a blank white bar (embed whose image Discord cannot fetch).

After: Discord now receives the image as an attachment instead of a blank embed. The sparse layout is from the local PIL renderer used only for this repro, not from the adapter change.

Verified locally against Discord (Astrbot#1126) with local T2I + file service (callback_api_base=http://127.0.0.1:6185). Existing tests/test_discord_adapter.py still pass.

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Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Fix Discord image delivery for loopback URLs by routing them through attachment handling while retaining embeds for public URLs.

Bug Fixes:

• Upload loopback image URLs as Discord file attachments so locally hosted images render correctly instead of appearing as blank embeds.

Enhancements:

• Preserve embedded-image handling for publicly reachable HTTP(S) URLs while recognizing localhost, loopback IPs, and IPv4-mapped loopback addresses.

Summary by Sourcery

Ensure Discord delivers locally hosted image URLs as file attachments while preserving embeds for public image URLs.

Bug Fixes:

• Upload images hosted on loopback or localhost URLs as Discord attachments so locally generated images render correctly instead of appearing as blank embeds.

Enhancements:

• Continue embedding publicly reachable HTTP(S) image URLs while recognizing localhost, loopback IPs, and IPv4-mapped loopback addresses.

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider extracting the loopback/localhost detection logic into a small helper function (e.g., is_loopback_host(url: str) -> bool) to keep _parse_to_discord easier to read and to centralize any future changes to this behavior.
• When checking for localhost and parsing IPs, it may be safer to normalize the hostname (e.g., lower()), and explicitly handle IPv6 literals with zone indices (like ::1%lo0) so they are consistently treated as unreachable loopback hosts rather than falling through to the embed path.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider extracting the loopback/localhost detection logic into a small helper function (e.g., `is_loopback_host(url: str) -> bool`) to keep `_parse_to_discord` easier to read and to centralize any future changes to this behavior.
- When checking for `localhost` and parsing IPs, it may be safer to normalize the hostname (e.g., `lower()`), and explicitly handle IPv6 literals with zone indices (like `::1%lo0`) so they are consistently treated as unreachable loopback hosts rather than falling through to the embed path.

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