fix(opencode): correct model fallback index tracking and config parsing

[manascb1344]

Fixes #1267

Summary

• Fix fallbackIndex never incrementing after successful model switch, causing infinite retry loop on same failing model
• Add models to knownKeys in config transform to prevent it being moved to options
• Add tests for edge cases: empty models array, options isolation, native agent fallbacks

What changed

processor.ts

• Increment fallbackIndex before calling tryFallbackModel, not after
• Simplified the helper to take targetIndex directly

config.ts

• Added "models" to the known keys set so the config transform preserves it correctly

agent.test.ts

• Added 3 new test cases covering edge cases

How I verified

• Ran bun test test/agent/agent.test.ts — all 31 tests pass
• Ran bun run typecheck — no errors
• Ran bun run test — 666 pass, 1 skip
• Reviewed the logic: each error now advances to the next fallback model instead of retrying the same one