refactor(docker): share container-name candidate iterator

Both prepare-time probing and launch-time race-retry generated the
same `<base>, <base>-2, ..., <base>-N` sequence with their own copies
of the suffix arithmetic and the 99-cap. Extract the candidate stream
into docker/util.container_name_candidates and have both call sites
walk it; each keeps its own predicate (probe vs. retry).

Also bumps the cap into a named constant (MAX_CONTAINER_SUFFIX) so
the two error messages can't drift.

claude_bottle/backend/docker/util.py

@@ -7,11 +7,25 @@ from __future__ import annotations
 import re
 import shutil
 import subprocess
-from typing import Iterable
+from typing import Iterable, Iterator
 
 from ...log import die, info
 
 
+# Cap on the suffix the container-name conflict logic will try before
+# giving up: base, base-2, ..., base-MAX_CONTAINER_SUFFIX.
+MAX_CONTAINER_SUFFIX = 100
+
+
+def container_name_candidates(base: str) -> Iterator[str]:
+    """Yield `base`, then `base-2`, `base-3`, ... up to
+    `base-MAX_CONTAINER_SUFFIX`. Both the prepare-time probe and the
+    launch-time race retry walk this sequence."""
+    yield base
+    for suffix in range(2, MAX_CONTAINER_SUFFIX + 1):
+        yield f"{base}-{suffix}"
+
+
 def runsc_available() -> bool:
     """Return True if the Docker daemon has the gVisor (`runsc`) runtime
     registered. Called once per prepare; the result lives on the plan."""