feat: UGraph/DGraph take vertex_class and edge_class

[petercorke]

Stacked on #30 (fix/connect-edge-endpoints): the base of this PR is that branch, so the diff shows only this change. It retargets to main when #30 merges.

Summary

A graph now owns the policy of which vertex and edge classes it builds, so user subclasses never have to be constructed by hand and passed in:

class MyVertex(UVertex): ...
class MyEdge(Edge): ...

g = UGraph(vertex_class=MyVertex, edge_class=MyEdge)
v1 = g.add_vertex([0, 0])      # a MyVertex
v2 = g.add_vertex([1, 1])
e = g.add_edge(v1, v2)         # a MyEdge, cost computed as usual

Defaults are unchanged (UVertex/DVertex and Edge), so existing code is unaffected.

Behavior

• vertex_class must subclass the graph's own vertex kind (UVertex for UGraph, DVertex for DGraph); edge_class must subclass Edge. Otherwise the constructor raises TypeError.
• add_vertex(v) only accepts instances of the graph's vertex_class; connect(edge=e) only accepts instances of its edge_class (TypeError otherwise). For a default graph this still admits any UVertex/DVertex/Edge subclass.
• Exposed as read-only g.vertex_class / g.edge_class; preserved by copy().
• UGraph.Dict() and UGraph.Adjacency() forward extra keyword arguments to the constructor.
• The classes are called as vertex_class(coord, name=name) and edge_class(v1, v2, cost=cost, data=data); this contract is in the constructor docstring.

For review

The strictness is the part most worth a second look: a graph configured with vertex_class=MyVertex rejects a plain UVertex. That is the point of configuring it (the type of g[i] can be relied on), but it is a choice.

Docstrings for UGraph, DGraph, Edge, connect() and the factories are updated, including the exceptions.

Tests

tests/test_graph_classes.py, 28 tests across UGraph and DGraph. mypy: no new errors.

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
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🟢 Metrics 35 complexity · 0 duplication

Metric	Results
Complexity	35
Duplication	0

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (53890d1) to head (fbb258d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   89.37%   90.31%   +0.94%     
==========================================
  Files           2        2              
  Lines         828      847      +19     
==========================================
+ Hits          740      765      +25     
+ Misses         88       82       -6     

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