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

[aiirvizionz]

Summary

• validate Cerebras custom baseUrl values before building the chat completions URL
• reject invalid URLs, non-http(s) schemes, credentials, query strings, and fragments before any network request
• keep trailing slash normalization covered while adding unsafe custom base URL cases

Why

The adapter trimmed trailing slashes, but still accepted malformed or surprising custom API bases. This change makes Cerebras fail early with adapter-specific configuration errors instead of sending requests to unclear destinations.

Validation

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

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.