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
     )