MTP: Extension node is a lie!

[chfast]

MPT: Remove explicit extended node kind

Notice that the formally specified extended node in
the Merkle Patricia Trie always leads to a branch node.
Therefore, we can treat branch nodes as having potentially non-empty
"extended path" and remove the explicit extended node kind.
This significantly simplifies the implementation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.29%. Comparing base (b1f68a3) to head (2314743).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #703   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files         143      143           
  Lines       16145    16110   -35     
=======================================
- Hits        15224    15191   -33     
+ Misses        921      919    -2     

Flag	Coverage Δ
eof_execution_spec_tests	16.47% <100.00%> (-0.16%)	⬇️
ethereum_tests	26.76% <100.00%> (-0.15%)	⬇️
ethereum_tests_silkpre	18.52% <100.00%> (-0.16%)	⬇️
execution_spec_tests	17.55% <100.00%> (-0.17%)	⬇️
unittests	89.72% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
test/state/mpt.cpp	100.00% <100.00%> (+1.81%)	⬆️

[chfast]

After extracting refactoring to #751, this is finally ready for review.

[Copilot]

Pull Request Overview

This PR simplifies the Merkle Patricia Trie (MPT) implementation by removing the explicit extended node type and treating branch nodes as having potentially non-empty extended paths instead.

• Removes the ext enum value from the Kind enum, changing it from uint8_t to bool
• Eliminates extended node creation methods and simplifies branch node construction
• Refactors the encoding logic to handle extended paths within branch nodes

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.14%. Comparing base (6b94215) to head (2d4b262).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #703      +/-   ##
==========================================
- Coverage   87.15%   87.14%   -0.01%     
==========================================
  Files         167      167              
  Lines       24751    24701      -50     
  Branches     4068     4063       -5     
==========================================
- Hits        21571    21525      -46     
+ Misses        541      538       -3     
+ Partials     2639     2638       -1     

Flag	Coverage Δ
eest-develop	17.59% <100.00%> (-0.16%)	⬇️
eest-develop-gmp	15.62% <97.14%> (-0.13%)	⬇️
eest-fusaka	12.23% <94.28%> (-0.09%)	⬇️
eest-legacy	11.13% <100.00%> (-0.15%)	⬇️
eest-legacy-silkpre	17.61% <97.14%> (-0.15%)	⬇️
eest-static	18.64% <97.14%> (-0.15%)	⬇️
evmone-unittests	83.70% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
core	93.30% <100.00%> (+<0.01%)	⬆️
tooling	88.35% <ø> (ø)
tests	84.10% <ø> (ø)

Files with missing lines	Coverage Δ
test/state/mpt.cpp	96.03% <100.00%> (+1.33%)	⬆️

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