fix(gateway): keep Discord docs out of photo vision routing

[LeonSGP43]

What does this PR do?

Fixes the mixed-attachment half of #25935. When a Discord message contains both images and documents, Hermes currently treats every attachment as an image if the message-level type is PHOTO. That causes non-image files like .md to be forwarded into the vision image path and can trigger provider-side Could not process image 400s.

This patch narrows the fallback: Hermes now trusts each attachment's own MIME type first, and only falls back to the message-level PHOTO classification when that attachment has no per-file MIME metadata.

Related Issue

Addresses #25935

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• Update gateway/run.py to classify inbound media per attachment, so document attachments on Discord photo messages no longer enter the image routing / vision path.
• Reuse the same per-attachment classification for busy-session media placeholders, so queued mixed attachments keep image/file distinctions.
• Add regressions in tests/gateway/test_native_image_buffer_isolation.py covering mixed image+document photo messages.

How to Test

1. Run python3 -m pytest -o addopts='' tests/gateway/test_native_image_buffer_isolation.py.
2. Verify a MessageType.PHOTO event with media_types=["image/png", "text/markdown"] only buffers the image path for native vision delivery.
3. Verify the busy placeholder renders the image attachment as an image and the .md attachment as a file, instead of marking both as images.

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 15.x

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — N/A
• I've updated cli-config.yaml.example if I added/changed config keys — N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — N/A

Screenshots / Logs

• python3 -m pytest -o addopts='' tests/gateway/test_native_image_buffer_isolation.py → 4 passed in 0.35s
• python3 -m py_compile gateway/run.py tests/gateway/test_native_image_buffer_isolation.py
• git diff --check
• Broader tests/gateway/test_discord_document_handling.py is red in this environment before/after the patch because current Discord attachment tests hit existing SSRF URL blocking against https://cdn.discordapp.com/attachments/fake/file; not caused by this change.

[alt-glitch]

Likely duplicate of #25936, which also fixes #25935 (per-attachment MIME classification to prevent docs from entering image routing). #25936 additionally adds image format transcoding.

[teknium1]

Merged via PR #53923 (commit 505bc27d) — #53923

Salvaged onto current main with both bugs fixed (mixed-attachment per-attachment routing + uncommon-format transcode), plus the document-context-note path widened so a doc mixed into a photo message reaches the agent as a readable file. Your work was combined with two other independent fixes for this issue and all three contributors are credited in the PR; @shashwatgokhe's authorship is preserved on the merge commit. Thanks for the fix.