Add durable task plan pause and resume

[EricSun0218]

Summary

• add non-terminal Paused task-plan status without changing existing enum values or JSON names
• add expectedRevision-checked pause/resume actions and explicit idempotent semantics
• retain the exact InProgress step across pause, restart, and resume
• keep paused plans visible and capacity-counted while excluding them from pending work
• reject every mutation except pause/resume while paused and keep terminal plans immutable
• publish pause/resume reasons and document migration/state-machine behavior

Validation

• locked restore passed
• full Release build: 48 projects, 0 warnings/errors
• full Release tests: 23 assemblies, 848/848 passed
• full dotnet format verification passed
• git diff --check passed