fix: resolve blob.GetAll random failures for blocks above tail

[DeVikingMark]

Problem

blob.GetAll method randomly fails to return blocks that are above the tail but within the prune window, causing intermittent errors in lightclient nodes.

Root Cause

Race condition in GetByHeight method where blocks below the current tail were immediately rejected without checking if they're still available in the store.

Solution

• Add fallback to check store directly before rejecting requests below tail
• Only reject if header is truly unavailable in store
• Maintain existing error behavior for genuinely unavailable blocks

Testing

• Added TestGetByHeightBelowTailButAvailable - verifies accessible blocks below tail are returned
• Added TestGetByHeightBelowTailNotAvailable - verifies truly unavailable blocks still return error
• All existing tests pass

Fixes #4603

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.37%. Comparing base (2469e7a) to head (8f6c32a).
⚠️ Report is 590 commits behind head on main.

Files with missing lines	Patch %	Lines
nodebuilder/header/service.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4611      +/-   ##
==========================================
- Coverage   44.83%   35.37%   -9.47%     
==========================================
  Files         265      304      +39     
  Lines       14620    24328    +9708     
==========================================
+ Hits         6555     8605    +2050     
- Misses       7313    14771    +7458     
- Partials      752      952     +200     

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