Remove MultiProcessor API

[karenychen]

Summary

• remove the unused NewMultiProcessor API and its ReceiverEx wrapper
• restore Processor to a single receiver while preserving retry, MaxReceiveCount, metrics, and panic recovery behavior
• remove MultiProcessor docs/examples/tests and keep single-processor panic recovery coverage

Verification

• go test ./... (from v2)
• go vet ./... (from v2)
• git diff --check
• e2e module compiled against local v2 via temporary modfile/replace

[karenychen]

Code review

No issues found. Checked for bugs and AGENTS.md compliance.