GraphParser: propagate parse errors as ArgumentException instead of swallowing

[wrigjl]

Summary

• Five methods in GraphParser.cs were catching all exceptions and silently returning empty lists, making parse failures invisible to callers (and producing incorrect results indistinguishable from valid empty output).
• Replaced each try/catch with explicit bounds checks that throw new ArgumentException(...) with a descriptive message and the offending parameter name, so bad input propagates as a 400 response.
• parseNodeListWithStringFunctions had a try/catch around string operations that cannot throw — the catch was removed entirely and the method reduced to a one-liner.
• Also clears 5 CS0168 warnings (unused catch variable e).

This really represents a poor design pattern. If there's an error, don't swallow it. Instead, figure out how to report it up the chain. Exceptions are a fine way of doing this.

Details per method

Method	Old behavior	New behavior
nodesGivenValidGraphAndPattern	swallow, return []	check eMatches.Count == 0, throw
getNodesFromNodeListString	swallow, return []	check both match collections, throw
parseNodeListWithStringFunctions	swallow, return []	no try/catch needed; one-liner
parseDirectedEdgeListWithStringFunctions	swallow, return []	check parts.Length < 2 per edge, throw with offending segment
parseUndirectedEdgeListWithStringFunctions	swallow, return []	check currentEdge.Length < 2 per edge, throw

Test plan

• dotnet build produces zero CS0168 warnings from GraphParser.cs
• Valid graph inputs parse correctly (no behavior change on the happy path)
• Invalid inputs return 400 rather than silently empty responses

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