xAI/Grok requests fail with 'grammar is too complex' when tool payloads are oversized #17405

Summary\nGrok requests can fail with grammar is too complex when the outbound tool payload is too large.\n\n## Repro\n- Use xai/grok-4 with a large toolset (many tools and/or large schemas).\n- Invoke opencode run so tools are attached to the request.\n- xAI returns grammar complexity errors.\n\n## Expected\nOpenCode should cap/prune xAI tool payloads before request dispatch so Grok does not receive over-budget grammar payloads.\n\n## Notes\nA pending PR adds hard tool count/payload caps and prunes the final tools map for xAI models.