feat: support apiKey arrays with round-robin rotation per provider

[gautamkrishnar]

Issue for this PR

Closes #29085

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds support for apiKey as an array of strings in provider options for round-robin rotation across multiple API keys.

When apiKey is an array in opencode.json:

{
  "provider": {
    "openrouter": {
      "options": {
        "apiKey": ["sk-key-1", "sk-key-2", "sk-key-3"]
      }
    }
  }
}

Each request rotates to the next key in the array (key1 → key2 → key3 → key1 → ...). This works for both the AI SDK provider path (via a fetch wrapper that swaps auth headers) and the native LLM path (via a new Auth.pool() credential that round-robins on each resolve). Single string apiKey continues to work identically — the array check only triggers for actual arrays.

I also added defensive type guards in Snowflake Cortex, Cloudflare Workers AI, and GitHub Copilot providers to prevent array coercion into malformed auth headers, and updated the v1 config schema to accept the new type.

How did you verify your code works?

• All 278 existing LLM package tests pass, plus 3 new tests for pool credential rotation
• Type checking passes across all 23 packages (core, llm, opencode, and all others via turbo)
• Tested live with Nvidia provider using two keys (one valid, one invalid) — confirmed round-robin alternation: valid key returns response, invalid key returns empty, next request uses valid key again

Screenshots / recordings

N/A — no UI changes

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[droid1-git]

Could you please reopen automatically closed PR.
Feature is needed.