Complete client library refactoring: fill all placeholders and implement missing functions

- Add REPLICATION_FACTOR and CHUNK_SIZE constants to config.h
- Add proper error codes (REJECTED, FULL, NOT_FOUND, TRANSFER_FAILED, etc.) to toastyfs.h
- Implement compute_chunk_hash() for deterministic chunk hashing
- Implement replay_request() to resend operations after leader redirect
- Implement send_message_to_server_ex() for sending messages with trailing data
- Implement choose_store_locations_for_chunk() to pick REPLICATION_FACTOR servers
- Fix begin_transfers() to handle both StoreChunk (PUT) and FetchChunk (GET)
- Complete toastyfs_async_put(): chunk splitting, hashing, data copy, metadata setup
- Complete toastyfs_async_get()/delete(): key setup, step/time initialization
- Fix get_result(): use actual transfer sizes instead of fixed chunk_size, free transfer data
- Replace all xxx placeholders and TOASTYFS_ERROR_XXX with proper values
- Add chunk_sizes[] tracking and put_data lifetime management
- Implement random_client.c: rng(), choose_random_oper(), proper API call arguments

https://claude.ai/code/session_01TkNavR4qyZcTuXMPmrtWUJ

include/toastyfs.h

@@ -13,6 +13,11 @@ typedef enum {
 typedef enum {
     TOASTYFS_ERROR_VOID,
     TOASTYFS_ERROR_OUT_OF_MEMORY,
+    TOASTYFS_ERROR_UNEXPECTED_MESSAGE,
+    TOASTYFS_ERROR_REJECTED,
+    TOASTYFS_ERROR_FULL,
+    TOASTYFS_ERROR_NOT_FOUND,
+    TOASTYFS_ERROR_TRANSFER_FAILED,
 } ToastyFS_Error;
 
 typedef struct {