Expose core public API from root package and fix broken prompt export

[Copilot]

Key classes like ChatAgent and ModelFactory were only importable from submodules, reducing discoverability for programmatic consumers. Additionally, camel.prompts.__all__ contained a non-existent name that caused ImportError at import time.

• camel/__init__.py: Re-export the 10 most-used classes (ChatAgent, ModelFactory, BaseMessage, ModelType, ModelPlatformType, FunctionTool, BaseToolkit, RolePlaying, Workforce, ChatAgentResponse) — aligned with the quickstart guide and 200+ examples
• camel/prompts/__init__.py: Remove phantom DescriptionVideoPromptTemplateDict from __all__ (only VideoDescriptionPromptTemplateDict exists)
• test/test_all_exports.py: Add parametrized test validating every __all__ entry resolves across 13 submodules to prevent regressions

# Before: required knowing submodule paths
from camel.agents import ChatAgent
from camel.models import ModelFactory
from camel.types import ModelPlatformType, ModelType

# After: also available from root
from camel import ChatAgent, ModelFactory, ModelPlatformType, ModelType

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