feat(proxy): enhance logging and deduplication for unknown datacenters

- Implemented a mechanism to log unknown datacenter indices with a distinct limit to avoid excessive logging.
- Introduced tests to ensure that logging is deduplicated per datacenter index and respects the distinct limit.
- Updated the fallback logic for datacenter resolution to prevent panics when only a single datacenter is available.

feat(proxy): add authentication probe throttling

- Added a pre-authentication probe throttling mechanism to limit the rate of invalid TLS and MTProto handshake attempts.
- Introduced a backoff strategy for repeated failures and ensured that successful handshakes reset the failure count.
- Implemented tests to validate the behavior of the authentication probe under various conditions.

fix(proxy): ensure proper flushing of masked writes

- Added a flush operation after writing initial data to the mask writer to ensure data integrity.

refactor(proxy): optimize desynchronization deduplication

- Replaced the Mutex-based deduplication structure with a DashMap for improved concurrency and performance.
- Implemented a bounded cache for deduplication to limit memory usage and prevent stale entries from persisting.

test(proxy): enhance security tests for middle relay and handshake

- Added comprehensive tests for the middle relay and handshake processes, including scenarios for deduplication and authentication probe behavior.
- Ensured that the tests cover edge cases and validate the expected behavior of the system under load.

src/proxy/masking.rs

@@ -232,6 +232,9 @@ where
     if mask_write.write_all(initial_data).await.is_err() {
         return;
     }
+    if mask_write.flush().await.is_err() {
+        return;
+    }
 
     let mut client_buf = vec![0u8; MASK_BUFFER_SIZE];
     let mut mask_buf = vec![0u8; MASK_BUFFER_SIZE];