TD-004: Unused isMultimodalMessageContent Function

[murdore]

Summary

The isMultimodalMessageContent type guard is exported but never used internally, creating dead code without clear documentation for external use.

Root Cause

• Function added but no internal usage created
• No documentation explaining it's for external integrators
• No code examples showing how to use it

Fix

Add comprehensive JSDoc with:

• @public tag marking it as external API
• Usage note explaining not used internally
• Two practical code examples
• Export from all compatibility layers

Acceptance Criteria

• [ ] Comprehensive JSDoc added
• [ ] Marked as @public
• [ ] Code examples added
• [ ] Tests verify function works

Files to Modify

• src/lib/types/multimodal.ts (lines 370-377)
• Create test/types/multimodal-message-content.test.ts

Dependencies

• Depends on: TD-003