Fix merge_message crash loop on nodes missing optional TRAPI fields

[maximusunc]

The merge_message worker was logging the same traceback dozens of times a second, on repeat, for the same two queries:

File "/app/shepherd_utils/shared.py", line 984, in merge_kgraph
  if value["name"]:

KeyError: 'name'

merge_kgraph subscripted name, categories and attributes on nodes and attributes/sources on edges. All of those are optional in TRAPI, and a subservice is free to omit one or send it as null -- is_support_edge right above already guards for exactly that on attributes. So one node without a name on an id already in the accumulator raised out of the process-pool child and took down the entire batch merge. Read them with .get() instead.

That alone was a bad callback; what made it a crash loop is the failure path. A failed merge re-enqueued its wake task immediately and unconditionally, and re-enqueueing mints a new stream message, so Redis' times_delivered (what the reclaim poison-pill breaker reads) resets to 1 every time and could never trip. A deterministic failure therefore spun: merge, raise, re-enqueue, with no backoff, for as long as the query lived -- burning a pool slot and never finishing the query.

So the failure path now backs off exponentially between attempts, carrying the attempt count in the task fields, and after merge_max_attempts (3) consecutive failures discards the batch it cannot merge -- clearing it from the ready index and callbacks table -- then re-enqueues with the counter reset so the query merges everything else and finishes. BrokenProcessPool goes through the same breaker: a batch that OOM-kills the child kills it again on every retry.

_clear_batch moves to module level alongside the new handler so both are unit-testable.

Claude-Session: https://claude.ai/code/session_011Ms8RqVffNEzs3X1Qz15To

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.91%. Comparing base (07f18b3) to head (c7e0954).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
workers/merge_message/worker.py	84.61%	3 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
shepherd_utils/config.py	95.53% <100.00%> (+0.12%)	⬆️
shepherd_utils/shared.py	80.84% <100.00%> (+0.91%)	⬆️
workers/merge_message/worker.py	70.40% <84.61%> (+1.79%)	⬆️

... and 1 file with indirect coverage changes

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