Support Polymorphic input payload deserialization (isolated dotnet)

[YunchuWang]

Fix #3182

This PR enables polymorphic deserialization of input payloads in the Durable Functions extension. In practical terms, it allows the system to accept inputs where the payload type isn’t known at compile time (or is one of several derived types) and properly deserialize them into the right concrete types rather than always using a base type or failing.

🧠 What the change addresses

Prior issue: Activities (especially in the isolated worker model) using e.g. Newtonsoft.Json or other serializers could not reliably handle input JSON that contained a type discriminator or represented a derived type of the declared parameter. This prevented polymorphic scenarios.

Issue describing the changes in this PR

resolves #issue_for_this_pr

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