refactor: Direct import of model representation to Python

[cqc-alec]

Closes #2287 .

Testing:

• All existing hugr-py tests that call validate() perform a round-trip, checking that the hashes of the start and end Hugrs (computed using the _NodeHash method defined in conftest.py) agree.
• I exported all the Hugrs generated by tests in the guppylang repo (using just export-integration-tests) and tried importing them (1) using the old method via json and (2) using the new method, and checked that the hashes of the two imported Hugrs agree (using the same _NodeHash method).

There are a few TODO comments remaining in load.py which I could not see a way to resolve using the existing model. I will investigate these further and raise issues if necessary.

[codecov]

Codecov Report

❌ Patch coverage is 69.06077% with 224 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.33%. Comparing base (1edeb25) to head (f3b2d12).

Files with missing lines	Patch %	Lines
hugr-py/src/hugr/model/load.py	70.09%	189 Missing ⚠️
hugr-py/src/hugr/tys.py	12.50%	14 Missing ⚠️
hugr-model/src/v0/binary/read.rs	0.00%	10 Missing ⚠️
hugr-py/src/hugr/model/__init__.py	76.31%	9 Missing ⚠️
hugr-py/src/hugr/envelope.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2683      +/-   ##
==========================================
- Coverage   83.53%   83.33%   -0.21%     
==========================================
  Files         266      267       +1     
  Lines       51718    52433     +715     
  Branches    47184    47194      +10     
==========================================
+ Hits        43204    43696     +492     
- Misses       6134     6357     +223     
  Partials     2380     2380              

Flag	Coverage Δ
python	88.60% <70.02%> (-2.93%)	⬇️
rust	82.75% <0.00%> (-0.02%)	⬇️

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