Add read-only mailbox pending queries

[EricSun0218]

Summary

• add typed batch mailbox pending-status queries for session/recipient keys
• distinguish ready messages from incomplete messages with active operational leases
• keep queries payload-free and free of lease, attempt, completion, model, or delivery side effects
• scan the file mailbox directory once per recipient batch and fail closed on corrupt state
• document scheduling and causal-boundary usage

Validation

• locked restore passed
• full Release build: 48 projects, 0 warnings/errors
• full Release tests: 23 assemblies, 842/842 passed
• full dotnet format verification passed
• git diff --check passed