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.

tests/unit/test_contrib_gitea_deploy_key.py

@@ -12,6 +12,7 @@ from bot_bottle.contrib.gitea.deploy_key_provisioner import (
     GiteaDeployKeyProvisioner,
     _split_owner_repo,
 )
+from bot_bottle.deploy_key_provisioner import DeployKeyCollisionError
 
 
 def _provisioner() -> GiteaDeployKeyProvisioner:
@@ -100,6 +101,30 @@ class TestCreate(unittest.TestCase):
                 provisioner.create("owner/repo", "title")
         self.assertIn("403", str(ctx.exception))
 
+    def test_create_raises_collision_error_on_422(self):
+        provisioner = _provisioner()
+        collision_body = json.dumps({
+            "errors": ["Key content already exists on this repository"],
+            "message": "422 Unprocessable Entity",
+        })
+        with patch(
+            "bot_bottle.contrib.gitea.deploy_key_provisioner.subprocess.run"
+        ), patch(
+            "bot_bottle.contrib.gitea.deploy_key_provisioner.urllib.request.urlopen",
+            side_effect=_http_error(422, collision_body),
+        ), patch(
+            "bot_bottle.contrib.gitea.deploy_key_provisioner.Path.read_bytes",
+            return_value=b"pk",
+        ), patch(
+            "bot_bottle.contrib.gitea.deploy_key_provisioner.Path.read_text",
+            return_value="ssh-ed25519 AAAA\n",
+        ):
+            with self.assertRaises(DeployKeyCollisionError) as ctx:
+                provisioner.create("owner/repo", "my-title")
+        msg = str(ctx.exception)
+        self.assertIn("owner/repo", msg)
+        self.assertIn("my-title", msg)
+
 
 class TestDelete(unittest.TestCase):
     def test_delete_calls_correct_endpoint(self):