fix(manifest): avoid KeyError when override bottle has repos absent from base

merge_bottles_runtime used .get(n, base_repos_by_name[n]) which eagerly
evaluates the default, crashing when a repo exists only in the override
bottle. Replaced with a conditional expression so the base lookup only
runs when needed.

Adds three regression tests covering override-only, base-only, and
name-collision cases.

Closes #457

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/manifest_extends.py

@@ -57,7 +57,7 @@ def _merge_two_bottles_runtime(base: "ManifestBottle", override: "ManifestBottle
         n for n in override_repos_by_name if n not in base_repos_by_name
     ]
     merged_git = tuple(
-        override_repos_by_name.get(n, base_repos_by_name[n])
+        override_repos_by_name[n] if n in override_repos_by_name else base_repos_by_name[n]
         for n in merged_repos_names
     )
 

tests/unit/test_manifest_bottle_merge.py

@@ -115,6 +115,30 @@ class TestMergeBottlesRuntime(unittest.TestCase):
         self.assertEqual("2", result.env["B"])
         self.assertEqual("3", result.env["C"])
 
+    def test_git_repo_only_in_override_does_not_raise(self):
+        # Regression for issue #457: override bottle declares a repo that the
+        # base doesn't have → KeyError on base_repos_by_name[n].
+        _repo = lambda url: {"url": url, "key": {"provider": "gitea", "forge_token_env": "TOK"}}
+        base = _bottle(env={"X": "base"})
+        override = _bottle(**{"git-gate": {"repos": {"myrepo": _repo("ssh://git@example.com/repo.git")}}})
+        result = merge_bottles_runtime([base, override])
+        self.assertIn("myrepo", [e.Name for e in result.git])
+
+    def test_git_repo_only_in_base_survives_override(self):
+        _repo = lambda url: {"url": url, "key": {"provider": "gitea", "forge_token_env": "TOK"}}
+        base = _bottle(**{"git-gate": {"repos": {"myrepo": _repo("ssh://git@example.com/repo.git")}}})
+        override = _bottle(env={"X": "override"})
+        result = merge_bottles_runtime([base, override])
+        self.assertIn("myrepo", [e.Name for e in result.git])
+
+    def test_git_repo_override_wins_by_name(self):
+        _repo = lambda url: {"url": url, "key": {"provider": "gitea", "forge_token_env": "TOK"}}
+        base = _bottle(**{"git-gate": {"repos": {"myrepo": _repo("ssh://git@base.example.com/repo.git")}}})
+        override = _bottle(**{"git-gate": {"repos": {"myrepo": _repo("ssh://git@override.example.com/repo.git")}}})
+        result = merge_bottles_runtime([base, override])
+        self.assertEqual(1, len(result.git))
+        self.assertEqual("ssh://git@override.example.com/repo.git", result.git[0].Upstream)
+
     def test_empty_list_raises(self):
         with self.assertRaises(ValueError):
             merge_bottles_runtime([])