fix(python): use the default context when none is configured

[rioyu123]

Summary

The Python toolkits currently try to call Configuration.Context.default() when Configuration.context is omitted or None. Context is a module-level class rather than an attribute of Configuration, so toolkit construction raises AttributeError before any PayPal request is made.

This change uses Context.default() in the OpenAI, LangChain, CrewAI, and Bedrock adapters. Caller-provided contexts and each adapter's source label remain unchanged.

Testing

• Compiled the Python package with compileall.
• Instantiated all four affected toolkits with context=None without making network requests and verified the default context and adapter-specific source.