fix: correct indentation bug in MsgBatchSigner._prepare_messages

[arewm]

Summary

• Fix incorrect indentation in MsgBatchSigner._prepare_messages that caused O(N²) message duplication when signing requests span multiple repositories
• Add test case to verify correct batching behavior with multiple repos

Problem

The run_in_parallel and messages.extend calls were incorrectly indented inside the repo loop. Since batch_data accumulates across all repos but was processed inside the loop, each repo iteration would re-process all accumulated batches.

Example with 3 repos (chunk_size=2, 3 digests per repo):

Repos	Buggy Messages	Fixed Messages	Duplication
1	2	2	1.0x
2	6	4	1.5x
3	12	6	2.0x
10	110	20	5.5x
100	10,100	200	50.5x

This was causing signing tasks to timeout in production when processing large numbers of images across multiple repositories.

Fix

Move the two affected lines outside the repo loop (dedent by 4 spaces):

             batch_data.append(fdata)

-            ret = run_in_parallel(self._create_msg_batch_message, batch_data)
-            messages.extend(list(ret.values()))
+        ret = run_in_parallel(self._create_msg_batch_message, batch_data)
+        messages.extend(list(ret.values()))
         return messages

Test plan

• Added test_prepare_messages_multi_repo_no_duplication test that:
  • Verifies run_in_parallel is called exactly once (not once per repo)
  • Verifies correct number of batches are created
  • Verifies each repo has the expected batch count

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (df19a66) to head (d0a90d0).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #58   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           24        24           
  Lines         1272      1272           
=========================================
  Hits          1272      1272           

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