[WIP] Add platform support feature for Python

[Copilot]

• Understand the blueprint specification in blueprint/api/platform.md
• Analyze existing provider implementations (OpenAI, Anthropic)
• Understand the AnyLLM base class and provider patterns
• Create py/src/any_llm/providers/platform.py with:
  • Platform key parsing (ANY.v1.kid.fingerprint-base64key format)
  • Key loading and public key derivation
  • Sealed box decryption with X25519 + XChaCha20-Poly1305
  • Challenge-response authentication flow
  • Token caching and management
  • Provider key retrieval and decryption
  • PlatformProvider class that wraps other providers
  • Usage tracking for API calls
• Register platform provider in py/src/any_llm/providers/__init__.py
• Add unit tests for platform provider (19 tests passing)
• Run linting and tests to verify implementation
• Run code review and address feedback:
  • Fixed minimum sealed box size check (48 bytes instead of 32)
  • Added descriptive error messages
  • Moved inline imports to top of file
• Run security scanning (CodeQL - 0 alerts)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Generation]: Platform support in Python</issue_title>
<issue_description>Implement the feature described in blueprint/platform.md for Python.

Put the output in py/providers/platform.py.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Generation]: Platform support in Python #767

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[daavoo]

@tbille for me the biggest miss is that the base AnyLLM.create was not updated to match how we do it in the original implementation:

any-llm/src/any_llm/any_llm.py

Lines 157 to 181 in 699dd54

	if api_key:
	try:
	# Validate if the key conforms with the any-api format.
	# If it does, any-llm must ask any-api for the corresponding provider key.
	PlatformKey(api_key=api_key)
	# Import and instantiate PlatformProvider in-place to avoid circular dependency issues.
	platform_class_name = "PlatformProvider"
	platform_module_path = "any_llm.providers.platform"
	try:
	platform_module = importlib.import_module(platform_module_path)
	except ImportError as e:
	msg = f"Could not import module {module_path}: {e!s}. Please ensure the provider is supported by doing AnyLLM.get_supported_providers()"
	raise ImportError(msg) from e
	platform_class: type[AnyLLM] = getattr(platform_module, platform_class_name)
	# Instantiate the class first and pass the provider next,
	# so we don't change the common API between different provideers.
	platform_provider = platform_class(api_key=api_key, api_base=api_base, **kwargs)
	platform_provider.provider = provider_class # type: ignore[attr-defined]
	except ValueError:
	pass
	else:
	return platform_provider

[tbille]

@daavoo can you add the blueprint for the platform provider in this PR?