fix(llm): allow registered custom providers in LlmConfig validation

[littfed]

Fixes #7625

Problem

LlmFactory.register_provider(name, class_path) allows callers to register custom or wrapping LLM providers into LlmFactory.provider_to_class. However, LlmConfig.validate_config in mem0/llms/configs.py checked provider against a hardcoded tuple of built-in providers. When constructing an LlmConfig or MemoryConfig with a registered custom provider, validation raised:
ValueError: Unsupported LLM provider: <name>

Solution

• Updated LlmConfig.validate_config to check provider in LlmFactory.provider_to_class dynamically instead of relying on a hardcoded tuple.
• Added unit tests in tests/utils/test_factory.py ensuring that registered providers pass LlmConfig validation while unsupported/unregistered providers continue to be rejected with a ValueError.

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[github-actions]

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[github-actions]

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