Adds asset tracking for explicit-path .md and .txt assets, and add cross-function coverage

[chalin]

• Contributes to Address site observability and data access #9559
• Adds asset-tracking edge function to cover explicit requests of .md and .txt assets
• Tracks direct asset requests regardless of response status and records the returned status_code and content_type
• Uses asset_group = text for explicit .txt assets such as llms.txt
• Adds unit coverage for .txt tracking and for status-preserving GA payloads
• Adds cross-function integration tests to ensure markdown negotiation and asset tracking queue only one GA event per request
• Updates routing, READMEs, and the asset analytics plan to match the current implementation
• Housekeeping: Renames gate-style unit tests to say returns false where they skip tracking rather than reject requests