fix(graph): support edge lookup via tuple key in GenericGraph.__getitem__

[sridhar-3009]

Summary

Fixes #3798.

GenericGraph.__getitem__ only looked up vertices. When a user passed a
(u, v) tuple to retrieve an edge mobject, it raised a KeyError against
self.vertices instead of looking up self.edges.

g = Graph([1, 2, 3, 4], [(1, 2), (2, 3)])
g[1]       # works — returns vertex mobject
g[(1, 2)]  # before: KeyError; after: returns edge mobject

Changes

manim/mobject/graph.py:

• Route 2-tuple keys to self.edges in __getitem__; all other keys
  continue to look up self.vertices

tests/module/mobject/test_graph.py:

• Add test_graph_getitem covering vertex lookup, edge lookup on
  Graph, and edge lookup on DiGraph

Test plan

• pytest tests/module/mobject/test_graph.py::test_graph_getitem

[chopan050]

Thanks for implementing this and adding tests! I left one comment which is necessary to address:

[chopan050]

One thing I mentioned in issue #3798 is that vertices can be any hashable object. This includes tuples: vertices can be tuples themselves. This is not too strange, actually. For example, vertices could represent 2D coordinates. There's also this concept of a "line graph L of a graph G" where each vertex of L is an edge of G: if there is an edge connecting vertices 1 and 3 of G, then L has a vertex (1, 3), and so on. networkx actually implements this.

It is possible that the graph contains two vertices, say, (1, 2) and (1, 3), and an edge connecting both: ((1, 2), (1, 3)). Thus, verifying that v is a tuple containing two elements is not enough to determine whether it represents an edge or a vertex.

One way I would solve this is by directly asking whether v is contained in the vertices or the edges before attempting to retrieve the Mobject:

if v in self.vertices:
    return self.vertices[v]
if v in self.edges:
    return self.edges[v]
raise IndexError(f"Vertex or edge {v} not found")
# or some more elaborated message, like
# "Couldn't find vertex {v} or edge connecting vertices {v[0]} and {v[1]}"
# when v is a tuple of 2 elements