fix: Inconsistent behaviour in SiblingSubgraph::from_nodes

[aborgna-q]

SiblingSubgraph::from_nodes and from_node have different definitions.

The former takes a set of nodes and defines a subgraph where the inputs and outputs correspond to all edges connecting the chosen nodes to external ones.
When a node has a disconnected port, it is ignored.

The latter does not check edges. It takes a single node and creates a subgraph using the dataflow signature of the operation.

Both definitions are valid and useful on their own, when calling from_nodes with a single-element vector we redirected the call to from_node, resulting in different port definitions.

This PR fixes that.

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.83%. Comparing base (f3dd145) to head (bad40dc).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/hugr/views/sibling_subgraph.rs	96.77%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2011   +/-   ##
=======================================
  Coverage   83.82%   83.83%           
=======================================
  Files         210      210           
  Lines       39645    39669   +24     
  Branches    36315    36339   +24     
=======================================
+ Hits        33232    33256   +24     
- Misses       4537     4538    +1     
+ Partials     1876     1875    -1     

Flag	Coverage Δ
python	92.01% <ø> (ø)
rust	83.08% <96.77%> (+0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #2011 will not alter performance

_{Comparing ab/sibling-subgraph-from-node (bad40dc) with main (f3dd145)}

Summary

✅ 21 untouched benchmarks

[doug-q]

Nice, thank you