Add Python dict headers overload to SubscriptionDataSource

[aiSynergy37]

Summary

• add a SubscriptionDataSource constructor overload that accepts Python dict headers (PyObject)
• convert Python dictionary headers into managed KeyValuePair<string, string> values
• reject non-dictionary Python objects with a clear ArgumentException
• add regression tests for valid, null, and invalid Python header inputs

Testing

• unable to run local tests in this environment because dotnet is not installed

Closes #9361

[AlexCatarino]

We have the ConvertToDictionary extension method we can use here:

 this(source, transportMedium, format, headers.ConvertToDictionary<string, string>())

By the way, we could use this method in QCAlgorithm.Python.cs#L1479 as well.

[aiSynergy37]

Done in d9548eb. I replaced the custom conversion with headers == null ? null : headers.ConvertToDictionary<string, string>() and updated the invalid-input test assertion to match the shared extension exception message.

[AlexCatarino]

Thank you, @aiSynergy37 ,
We should reuse the ConvertToDictionary extension method.

[aiSynergy37]

Addressed review feedback in d9548eb: the PyObject headers overload now reuses ConvertToDictionary<string, string>() instead of custom conversion logic, and tests were updated accordingly.

[aiSynergy37]

@AlexCatarino Thanks again. The ConvertToDictionary refactor is now in place on the latest commit (d9548eb). Could you please take another look when you have a moment?