fix(pluto): improve onlyOne() error message with diagnostic context
The onlyOne() method in Pluto threw a generic Error("something wrong") with no context about what was being looked up or why it failed.
Changes:
- Accept optional context string to identify the caller and query
- Distinguish between empty results (arr.length === 0) and null item
- Report the actual count when multiple results found
- Pass context from getPairByDID() and getPairByName() callers
Fixes #648
Signed-off-by: A-Chronicle <[email protected]>