feat: expose fluid communities to python

[BeaMarton13]

I used Cody for the functions.

• By submitting this pull request, I assign the copyright of my contribution to The igraph development team.

[szhorvat]

This looks great to me @BeaMarton13, other than the small nitpicks I noted!

@ntamas, can you also have a look please?

[ntamas]

LGTM, apart from what Szabolcs already noted.

[Copilot]

Pull Request Overview

This PR adds support for fluid communities in the Python interface of igraph.

• Implements the fluid communities algorithm in the C and Python layers.
• Updates the igraph API (init.py) to expose the new feature.
• Includes several tests in tests/test_decomposition.py validating the expected behavior and error conditions.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/test_decomposition.py	Added test cases for the fluid communities algorithm.
src/igraph/community.py	Introduced the Python wrapper for the fluid communities method.
src/igraph/init.py	Exposed the new fluid communities API in igraph’s public interface.
src/_igraph/graphobject.c	Added the C binding for fluid communities.

[Copilot]

Consider catching a more specific exception type (e.g., ValueError) instead of the generic Exception for better clarity in testing error conditions.

Suggested change

	with self.assertRaises(Exception):
	g.community_fluid_communities(0)
	# Number of communities cannot exceed number of vertices
	with self.assertRaises(Exception):
	g.community_fluid_communities(g.vcount() + 1)
	# Test with disconnected graph (should raise error)
	g_disconnected = Graph.Full(3) + Graph.Full(3) # No connecting edge
	with self.assertRaises(Exception):
	with self.assertRaises(ValueError):
	g.community_fluid_communities(0)
	# Number of communities cannot exceed number of vertices
	with self.assertRaises(ValueError):
	g.community_fluid_communities(g.vcount() + 1)
	# Test with disconnected graph (should raise error)
	g_disconnected = Graph.Full(3) + Graph.Full(3) # No connecting edge
	with self.assertRaises(ValueError):

[szhorvat]

@ntamas This is actually an InternalError. Should we assert that? How does that Python interface determine what becomes a ValueError? Note that this is indeed an IGRAPH_EINVAL.