Adopted from #3705 by @fangxueshun with fixes:
- Added input validation for date strings (RFC 3339)
- Used chrono DateTime comparison instead of string comparison
- Added since < until validation
- Updated mem0 backend
Supersedes #3705
Replace `tokio::task::spawn_blocking()` with plain `std:🧵:Builder`
OS threads in all PostgresMemory trait methods. The sync `postgres` crate
(v0.19.x) internally calls `Runtime::block_on()`, which panics when called
from Tokio's blocking pool threads in recent Tokio versions. Plain OS threads
have no runtime context, so the nested `block_on` succeeds.
This matches the pattern already used in `PostgresMemory::initialize_client()`,
which correctly used `std:🧵:Builder` and never exhibited this bug.
A new `run_on_os_thread` helper centralizes the pattern: spawn an OS thread,
run the closure, and bridge the result back via a `tokio::sync::oneshot` channel.
Fixes#4101
