core: return explicit error on AddPartWithoutProof ok=false in partsToBlock

[Forostovec]

This change fixes incorrect error handling in partsToBlock where a false ok result from PartSet.AddPartWithoutProof led to returning nil, err with err being nil, effectively masking failures such as duplicate or otherwise non-added parts. By returning a concrete error with context (part index) when ok is false, we ensure block reconstruction errors are surfaced to callers, aligning with CometBFT semantics that reserve non-nil err for validation failures and use ok=false for non-added cases (commonly duplicates). This prevents silent success, improves diagnosability, and maintains correct behavior in higher-level consumers like receiveBlockByHeight/receiveBlockByHash, Listener, and Exchange.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 35.16%. Comparing base (2469e7a) to head (4ccb973).
⚠️ Report is 596 commits behind head on main.

Files with missing lines	Patch %	Lines
core/fetcher.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4644      +/-   ##
==========================================
- Coverage   44.83%   35.16%   -9.67%     
==========================================
  Files         265      304      +39     
  Lines       14620    24307    +9687     
==========================================
+ Hits         6555     8548    +1993     
- Misses       7313    14798    +7485     
- Partials      752      961     +209     

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