feat: add Astraflow provider support

[ucloudnb666]

What type of change does this PR introduce?

• Bugfix
• Feature
• Refactor
• Documentation
• Not Sure?

Does this PR introduce breaking changes?

• Yes
• No

List any relevant issue numbers:

Description:

Adds Astraflow (by UCloud / 优刻得) as a supported LLM provider. Astraflow is an OpenAI-compatible AI model aggregation platform supporting 200+ models.

• Global endpoint: https://api-us-ca.umodelverse.ai/v1
• Website: https://astraflow.ucloud-global.com
• API key signup: https://astraflow.ucloud-global.com

Changes:

File	Change
server/src/core/llm-manager/types.ts	Added Astraflow = 'astraflow' to LLMProviders enum
server/src/core/llm-manager/llm-provider.ts	Registered astraflow-llm-provider in LLM_PROVIDERS_MAP
server/src/core/llm-manager/llm-providers/astraflow-llm-provider.ts	New provider file using AISDKRemoteLLMProvider with flavor: 'openai-compatible'
server/src/core/llm-manager/llm-provider-account-configs.ts	Added Astraflow account config with LEON_ASTRAFLOW_API_KEY
.env.sample	Listed astraflow in providers comment; added LEON_ASTRAFLOW_API_KEY variable

Usage:

LEON_ASTRAFLOW_API_KEY=your_api_key_here

LEON_LLM=astraflow/your-model-name

The integration follows the same pattern as Z.AI and other OpenAI-compatible providers, using flavor: 'openai-compatible' in the AISDKRemoteLLMProvider base class.