Fix crash when referencing non-existent target ref ID

[Nepomuk5665]

Summary

• Fixes a runtime crash (TypeError) when a graph target references a non-existent ref ID
• Adds proper error handling with a descriptive error message
• Adds test coverage for the fix

Problem

The handleRefTargets function in src/panels/graph.ts used Array.find() with a TypeScript non-null assertion (!) to look up target references by refId:

const findTargetByRefId = (targets: TargetObject[], refId: string) =>
    targets.find((target) => target.refId === refId)!.target;

When a target referenced a non-existent ref (e.g., #Z when only #A and #B exist), find() returned undefined and accessing .target on it caused:

TypeError: Cannot read property 'target' of undefined

Solution

Added explicit null checking with a descriptive error message that includes:

• The invalid ref that was referenced
• All available refs for easy debugging

Example error message:

Error: Invalid target reference: #Z does not exist. Available refs: A, B

Testing

• Added a new test case that verifies the descriptive error is thrown
• All existing tests pass

[CLAassistant]

All committers have signed the CLA.