fix(deploy-key): raise DeployKeyCollisionError on 422 key conflicts

Gitea returns HTTP 422 when a deploy key title or public key content
already exists on the repo. The provisioner previously surfaced this
as a generic RuntimeError with the raw status code. Introduce
DeployKeyCollisionError (a RuntimeError subclass) in the base module
and detect 422 in GiteaDeployKeyProvisioner.create so callers can
catch collisions explicitly and the error message names the repo and
title involved.

bot_bottle/deploy_key_provisioner.py

@@ -11,6 +11,10 @@ from __future__ import annotations
 from abc import ABC, abstractmethod
 
 
+class DeployKeyCollisionError(RuntimeError):
+    """Raised when a deploy key title or public key already exists on the repo."""
+
+
 class DeployKeyProvisioner(ABC):
     """Manages a single deploy-key lifecycle on a remote forge."""