UN-2813 [FIX] Ensure backend config returns correct backend_type

Fix critical bug where get_backend_config_for_type() returned configs
with incorrect backend_type field.

**Problem:**
When calling get_backend_config_for_type("hatchet") or "temporal", the
returned config object had backend_type="celery" (inherited default from
BaseWorkerConfig) instead of the requested type. This caused CLI commands
like `--backend=temporal` to spin up the wrong backend.

**Solution:**
- Refactored to use config class map
- Pass backend_type explicitly to config constructor
- Added proper exception chaining with `from None`

**Impact:**
- CLI backend selection now works correctly
- Config objects have correct backend_type field
- Prevents subtle bugs from mismatched backend types

**Example:**
```python
# Before: config.backend_type = "celery" ❌
config = get_backend_config_for_type("temporal")

# After: config.backend_type = "temporal" ✅
config = get_backend_config_for_type("temporal")
```

Resolves: CodeRabbit comment #2386596266
Related: #1555

Author: Deepak-Kesavan

task-backend/src/unstract/task_backend/config.py

@@ -133,11 +133,14 @@ def get_task_backend_config() -> TaskBackendConfig:
 
 def get_backend_config_for_type(backend_type: str) -> TaskBackendConfig:
     """Get configuration for a specific backend type."""
-    if backend_type == "celery":
-        return CeleryWorkerConfig()
-    elif backend_type == "hatchet":
-        return HatchetWorkerConfig()
-    elif backend_type == "temporal":
-        return TemporalWorkerConfig()
-    else:
-        raise ValueError(f"Unsupported backend type: {backend_type}")
+    config_map = {
+        "celery": CeleryWorkerConfig,
+        "hatchet": HatchetWorkerConfig,
+        "temporal": TemporalWorkerConfig,
+    }
+    try:
+        config_cls = config_map[backend_type]
+    except KeyError:
+        raise ValueError(f"Unsupported backend type: {backend_type}") from None
+
+    return config_cls(backend_type=backend_type)