feat: add RecipientNotFoundError exception for unregistered agent recipients

[Br1an67]

Why are these changes needed?

Currently, when send_message() is called with an unregistered AgentId, SingleThreadedAgentRuntime raises a generic Exception("Recipient not found"). This makes it impossible for users to specifically catch and handle this error without catching all exceptions.

This PR introduces a dedicated RecipientNotFoundError exception class that:

• Allows precise except RecipientNotFoundError handling
• Includes the recipient AgentId in the error message for easier debugging
• Follows the existing exception pattern in autogen_core.exceptions (CantHandleException, UndeliverableException, etc.)

Related issue number

Closes #4964

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

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[Br1an67]

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