feat(discord): add proxy support and tests

[nayihz]

📝 Description

Add Discord proxy support for both REST and websocket paths, with coverage tests. This ensures Discord can work in proxied environments when a channel proxy is configured (including socks5://) and improves behavior around proxy setup validation.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

N/A

📚 Technical Context (Skip for Docs)

• Reference URL: N/A
• Reasoning: Route Discord HTTP client and websocket dialer through configured proxy logic, and add tests to prevent regressions.

🧪 Test Environment

• Hardware: PC
• OS: macOS
• Model/Provider: N/A
• Channels: Discord

📸 Evidence (Optional)

Click to view Logs/Screenshots

Added pkg/channels/discord_test.go and validated proxy setup behavior via unit tests.

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.