feat: add LMDB unit tests for BlockchainBackend read methods

Create comprehensive unit tests that exercise LMDB read operations against
a realistic blockchain containing a fork and reorg.

Test chain topology:
  Genesis -> B1..B5 -> B6..B10 (original)
                   \-> F6'..F15' (fork, triggers reorg)

After reorg: canonical = Genesis + B1..B5 + F6'..F15'
Reorged blocks: B6..B10

Tests cover:
- Chain construction and reorg verification
- JSON serialization for reproducibility
- fetch_outputs_in_block / fetch_output / fetch_unspent_output_hash_by_commitment
- fetch_inputs_in_block
- fetch_kernels_in_block / fetch_kernel_by_excess_sig
- fetch_header_containing_kernel_mmr
- fetch_outputs_in_block_with_spend_state
- fetch_mined_info_by_payref

22 test functions organized in write_tests and read_tests modules.

Fixes #7715

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: 0xPepeSilvia