ledger: clarify ShredData comments (no zero padding, remove legacy mention)

[GarmashAlex]

• bytes_to_store: now states it returns the canonical payload for blockstore.
• resize_stored_shred: clarified it only validates Merkle Data payload size and returns it unchanged; it does not perform zero padding.
• capacity: removed outdated “None indicates a legacy data-shred” note and clarified that proof_size is the number of Merkle proof entries and resigned indicates presence of a retransmitter signature.

The previous comments were misleading: they suggested possible zero-padding (which is not implemented) and referenced a legacy Option-based API that no longer exists.

[0xbrw]

@alexpyattaev are able to take a look at this?

[alexpyattaev]

Thanks for looking at this, these comments are remnants of the legacy shred removal.

To be frank I think we can axe the entire function, as it is essentially a noop (as shred length should be validated during deserialization). @steviez wdyt?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.1%. Comparing base (1cc3011) to head (d43b266).
⚠️ Report is 24 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8426   +/-   ##
=======================================
  Coverage    83.1%    83.1%           
=======================================
  Files         838      838           
  Lines      367672   367672           
=======================================
+ Hits       305891   305903   +12     
+ Misses      61781    61769   -12     

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