fix(im): handle unresolved Feishu resource images

[Jonesxq]

Description

Feishu interactive cards reject unresolved resource:// Markdown images as invalid image keys. The adapter's image matcher only handled HTTP(S), so unresolved resource handles passed through unchanged and could make the entire card update fail.

This change:

• recognizes resource:// image targets in resolveMarkdownImages;
• degrades unresolved resource images to ordinary Markdown links using the existing regional fallback label;
• keeps the existing HTTP(S) upload-to-image_key path unchanged.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

Fixes #2726

Testing

go test ./internal/im/feishu -count=1
go test ./internal/im/... -count=1
go vet ./internal/im/feishu
golangci-lint run --new-from-rev=origin/main ./internal/im/feishu

All commands pass; golangci-lint reports 0 issues.

The full repository suite was not run because the change is isolated to the Feishu adapter; the complete internal/im/... package tree was tested.

Checklist

• git diff --check origin/main...HEAD passes
• Changed source files are formatted
• Targeted tests for the changed packages/components pass
• Diff-scoped lint passes where applicable
• Full-repository checks were run, or any unrelated/environment-dependent limitations are documented above
• Self-reviewed the code
• Added tests covering the change
• Documentation is not affected; no public API or configuration changed
• No breaking changes

Screenshots / Recordings

Not applicable; this is an IM adapter behavior fix.

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