provider: add SaladCloud provider

[mgorkii-nlplogix]

Description:

• Added SaladCloud as an OpenAI-compatible provider for chat completions.
• Registered saladcloud as a valid provider and added provider metadata.
• Added SaladCloud API routing for https://ai.salad.cloud/v1/chat/completions.
• Added SaladCloud test variables using SALADCLOUD_API_KEY or SALAD_CLOUD_API_KEY.
• Added a focused provider API config test.

Tests Run/Test cases added: (required)

• Added src/providers/saladcloud/index.test.ts to verify SaladCloud base URL, chat completions endpoint, and authorization header.
• Ran npx jest --runTestsByPath src/providers/saladcloud/index.test.ts.
• Ran npm run format:check.
• Ran npm run build.
• Verified live non-streaming SaladCloud chat completion request returns 200.
• Verified live streaming SaladCloud chat completion request returns SSE chunks and [DONE].

Type of Change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)