fix(ai-nebius): require clean api base urls

[aiirvizionz]

Summary

• normalize Nebius custom baseUrl values before appending /v1/chat/completions
• reject invalid URLs, non-http(s) schemes, credentials, query strings, and fragments before any network request
• extend tests for trailing slash normalization and unsafe custom base URLs

Why

The adapter currently concatenates user-provided baseUrl directly into the request URL. That can produce malformed request paths or surprising destinations. This change makes Nebius fail early with adapter-specific configuration errors.

Validation

• vitest run packages/ai/nebius/src/index.test.ts
• tsc -p packages/ai/nebius/tsconfig.json --noEmit
• git diff --check

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