Improve tool answer fallback synthesis

[asim]

Summary

• Replace generic incomplete-answer copy with a compact synthesized fallback from successful tool results.
• Keep unavailable tool slices explicit when some live data is missing.
• Add regression coverage for organized fallback output and unavailable slice naming.

Closes #773
Closes #777

Testing

• go build ./...
• go test ./... -short
• go vet ./...