refactor(blockchain): change default transaction layout to combined and remove related flags from commands

[thiagodeev]

Summary

• Makes the blocktransactions migration mandatory (no longer opt-in) and switches the default transaction storage layout from per-tx to combined (per-block)
• Removes the --transaction-combined-layout CLI flag and all associated config/wiring
• Returns db.ErrKeyNotFound for non-existent blocks in combined layout (consistent error propagation), and adds test coverage for both non-existent blocks and valid
  blocks with no transactions

A follow-up PR will be made to completely remove the old layout code.

Changes

Migration & config cleanup:

• node/migration.go: WithOptional → With for the blocktransactions migrator
• node/node.go: Remove FlagTransactionCombinedLayout constant and TransactionCombinedLayout from Config
• blockchain/blockchain.go: Change default layout in New() to TransactionLayoutCombined
• cmd/juno/juno.go + cmd/juno/dbcmd.go: Remove --transaction-combined-layout flag from all commands

Tests:

• blockchain/blockchain_test.go: Add test for valid block with no transactions (expects success + empty result), and update non-existent/reverted block tests to expect errors
• core/transaction_layout_test.go: Add NonExistentBlock_* subtests verifying both layouts return errors for missing blocks
• Some cases in the TestTransactionLayout are being skipped for the old layout since it behaves differently, and we plan to remove it

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.28%. Comparing base (499b208) to head (6774432).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
node/migration.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3506      +/-   ##
==========================================
- Coverage   75.48%   75.28%   -0.21%     
==========================================
  Files         386      386              
  Lines       34907    34912       +5     
==========================================
- Hits        26350    26283      -67     
- Misses       6712     6728      +16     
- Partials     1845     1901      +56     

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[rodrodros]

Looks good!