Fix local plan cache growth for prepared statements

[Hamid Akhtar (hqakhtar)]

PR #8371 made cache lookup reject plans whose task list did not contain exactly one task. Deferred-pruning plans keep the persistent plan's task list empty, so every execution missed the existing local plan and added another one. This caused unbounded memory growth.

Separate cache eligibility from lookup. Use the current job for eligibility and the persistent distributed plan for cache storage. Keep the multi-shard reuse guard and allocate cache entries in the persistent plan's memory context.

Add a regression helper that reads the generic plan's cache size. Verify that repeated single-shard executions retain one entry and a multi-shard execution does not add another.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.72%. Comparing base (92d0b92) to head (0a10bdd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8824      +/-   ##
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- Coverage   88.73%   88.72%   -0.02%     
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  Files         289      290       +1     
  Lines       65013    65068      +55     
  Branches     8203     8209       +6     
==========================================
+ Hits        57691    57730      +39     
- Misses       4954     4963       +9     
- Partials     2368     2375       +7     

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