feat(discord): feature-complete webhook provider with embeds, base64 file attachments, edit, rate-limit retry, host restriction, and guardrails

[aries1980]

Summary

Feature-completes the Discord webhook provider against the Execute Webhook + Edit Message APIs:

• Generic JSON passthrough — embeds, username, avatar_url, allowed_mentions, flags, thread_name, applied_tags, poll, attachments forwarded as-is
• File attachments via base64 — files param via multipart/form-data; no filesystem paths supported (security rationale in code/docs); accepts bytes, (filename, content, content_type) tuples, or {"base64": "...", "filename": "..."} dicts
• Message editing — message_id param switches to PATCH .../messages/{id}; snowflake-validated
• Rate-limit retry — bounded 3-attempt retry on 429 with Retry-After (clamped to 30s max)
• webhook_url host restriction — only discord.com/discordapp.com accepted
• validate_scopes() — GET on the webhook URL to confirm it is real and reachable
• Client-side guardrails — content ≤2000 chars, ≤10 embeds, ≤10 files, ≤10 MiB per file
• tts and delete deliberately dropped — marginal value, not worth surface area

New: 53 tests

Documentation updated

• Provider MDX page, auto-generated snippet, README, example workflow

Closes #6645

[aries1980]

For the ruff-related errors, I created a mega PR: #6649

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.47%. Comparing base (98dbd7e) to head (3d21358).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6646      +/-   ##
==========================================
- Coverage   30.49%   30.47%   -0.03%     
==========================================
  Files         101      101              
  Lines       11741    11770      +29     
==========================================
+ Hits         3581     3587       +6     
- Misses       8160     8183      +23     

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[aries1980]

@shahargl : Hi, after the rebase, the issues went away, I think it is ready for review.