Provider preset: FriendliAI (serverless)

[frostyhand]

Any OpenAI-compatible provider already works via custom; this issue is the one-line nickname.

Verified 2026-07-12:

• Base URL: https://api.friendli.ai/serverless/v1 (serverless plane; dedicated endpoints use a different base)
• Keyless smoke: POST {base}/chat/completions returns 401 {"message":"Unauthorized. Please provide a valid credential."}
• Docs: https://friendli.ai/docs/guides/serverless_endpoints/openai-compatibility
• Example model: Qwen/Qwen3-235B-A22B-Instruct-2507 (HF-style id with a slash; worth one test asserting the model id is not path-mangled)

Acceptance: one entry in src/router/upstream.ts PRESETS with a comment noting the serverless vs dedicated split, README provider table row, tests green.