feat: Export entrypoint metadata in Python and fix bug in import

[zrho]

This PR ensures that the core.entrypoint metadata is exported in the Python bindings. It also fixes a bug in hugr-core import that added children of the root node to the entrypoint instead of the root.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.56%. Comparing base (6a0e270) to head (41480bd).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/import.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2434   +/-   ##
=======================================
  Coverage   82.56%   82.56%           
=======================================
  Files         247      247           
  Lines       45894    45900    +6     
  Branches    41528    41528           
=======================================
+ Hits        37891    37897    +6     
  Misses       5981     5981           
  Partials     2022     2022           

Flag	Coverage Δ
python	91.26% <100.00%> (+0.01%)	⬆️
rust	81.64% <0.00%> (ø)

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[ss2165]

Either split in to two PRs or include both changes in PR title (otherwise second is lost in changelog)

[zrho]

Edited the title since it makes sense to keep them together: the metadata export from Python unearthed the bug.

[ss2165]

I don't understand the behaviour of the standalone arg. A docstring addition might help.

[zrho]

The standalone argument was meant to prevent the entrypoint metadata to be emitted twice when a core node corresponds to both a node and a region in hugr-model (which is the case for all hierarchical nodes except for Case). The argument is gone now, since I remembered that we wanted the regions to be the entrypoints and not the nodes.

[ss2165]

thanks