Python: Clarify allowed types for deserialization

[XananasX7]

1. Why is this change required?
   The previous implementation of _RestrictedUnpickler contained a critical security flaw that allowed Remote Code Execution (RCE). It whitelisted entire module namespaces (agent_framework.* and openai.types.*), which is inherently unsafe in Python's pickle deserialization.

2. What problem does it solve?
   It prevents "gadget" attacks. By whitelisting a whole namespace, an attacker could use any class within the framework that has side effects during initialization (e.g., executing commands, opening files) to compromise the host. This change enforces a strict, explicit whitelist policy.

3. What scenario does it contribute to?
   Secure handling of Durable Workflows and Checkpoints. It ensures that when agents load state from potentially untrusted sources (shared storage, databases, or user uploads), the system remains protected against malicious payloads.

4. If it fixes an open issue, please link to the issue here.
   Security vulnerability identified via independent audit (MSRC report pending).

Description
This PR hardens the _RestrictedUnpickler in _checkpoint_encoding.py by:

Removing over-broad whitelists: Deleted the logic that automatically allowed any module starting with agent_framework. or openai.types..

Enforcing Explicit Whitelisting: The unpickler now strictly only allows types defined in _BUILTIN_ALLOWED_TYPE_KEYS or those explicitly provided by the user via the allowed_checkpoint_types parameter.

Updating Documentation: Updated the module and method docstrings to clearly state that callers are now responsible for explicitly whitelisting any framework or custom classes they intend to deserialize.

Contribution Checklist
[x] The code builds clean without any errors or warnings

[x] The PR follows the Contribution Guidelines

[x] All unit tests pass, and I have added new tests where possible

[x] Is this a breaking change? Yes. [BREAKING] (Callers must now explicitly whitelist framework types they use in checkpoints).

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