fix(chat): delete persisted conversation data

[geyaovip]

Summary

• delete persisted LangGraph checkpoints even when the agent is stopped, using the configured SQLite or PostgreSQL memory backend
• make dashboard and CLI deletion remove checkpoint data before thread metadata, so a failed cleanup remains visible and retryable
• show Octop-native success/failure messages only after deletion finishes, without adding a visible loading state to the delete action
• cover running/stopped agents, real checkpoint storage, API failure retention, CLI deletion, and dashboard feedback with tests

Verification

• make all: 3668 passed, 17 skipped
• pre-commit hook: 3685 passed, 50 skipped; dashboard production build passed
• focused backend tests after rebasing onto latest develop: 131 passed, 1 skipped
• dashboard hook tests: 3 passed
• npx tsc -b: passed
• isolated copy of a real checkpoint database: thread checkpoint rows changed from 16 to 0 after deletion

Addresses #725.

[Bosheng0422]

Thx for the PR, seems not rly solve the issue #725