fix(core): Return partial response for headers if one fails

[renaynay]

Fixes an issue where we would discard all headers in case one failed. Now we can at least return the ones we synced successfully (as long as they're contiguous from from) so we don't need to do duplicate work.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 36.00%. Comparing base (2469e7a) to head (4b30d03).
⚠️ Report is 633 commits behind head on main.

Files with missing lines	Patch %	Lines
core/exchange.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4738      +/-   ##
==========================================
- Coverage   44.83%   36.00%   -8.83%     
==========================================
  Files         265      307      +42     
  Lines       14620    20777    +6157     
==========================================
+ Hits         6555     7481     +926     
- Misses       7313    12326    +5013     
- Partials      752      970     +218     

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[walldiss]

A few concerns:

Syncer assumes full range on nil error. In go-header's sync/syncer.go:373, requestHeaders does amount -= size (the requested size, not len(headers)) and advances fromHead to the last returned header. If this returns fewer headers with nil error, the syncer will silently skip the gap between what it got and what it thinks it got. There's also a panic if the partial result is empty (headers[len(headers)-1]).

Concurrent fill makes nil-scan unreliable. errgroup.WithContext cancels remaining goroutines on first error. Earlier slots may still be nil if those goroutines hadn't written yet. The contiguous prefix assumption only holds reliably when failures are at the tail (e.g. beyond chain tip), not for mid-range transient errors.

h.IsZero() is redundant — it's just eh == nil, same as the nil check before it.

Suggestion: either return the error alongside partial results (new sentinel like ErrPartialRange so callers can opt in), or fix the syncer in go-header to use amount -= uint64(len(headers)) before landing this.

[vgonkivs]

Moving it to draft until go-header pr merged and tested. @renaynay do you want to finish it then or should I take it over ?