fix(mpp): reject non-string opaque values

[cryptoryda]

Reject non-string opaque metadata values

Summary

Validate that every value in challenge and credential opaque metadata is a string.

The MPP opaque field is a flat string-to-string object. Previously, non-string values such as numbers, booleans, arrays, and nested objects were passed through anyStr, which could silently coerce them or turn them into empty strings.

Changes

• Add a shared conversion helper that rejects non-string map values.
• Apply strict validation when parsing challenge opaque metadata.
• Apply the same validation to encoded-string and object credential representations.
• Reject unsupported credential opaque representations.
• Add public parser regression tests for each affected representation.

Testing

• go test ./pkg/mpp
• go vet ./...
• go test -race -count=1 ./...