feat: add cost-aware vision routing example

[z116123123x]

What

Adds examples/vision_routing_example.py demonstrating covered-first vision routing for Z.ai GLM models.

Why

Current README examples only use glm-5v-turbo (the paid model). Users with $0 cash balance hit HTTP 429 / 1113 Insufficient balance with no guidance that the covered (free) glm-4.6v route exists.

This example shows:

• Default covered-first routing (free model)
• Opt-in cash fallback with ZAI_VISION_ALLOW_CASH=1
• Per-run cost guard via ZAI_VISION_CASH_MAX_CALLS_PER_RUN
• Refusal detection and retry logic

Verification

• python examples/vision_routing_example.py runs standalone (no external deps)
• 100 lines, follows existing example style

[z116123123x]

Closing this while I restart the contribution assessment from scratch. I want to re-evaluate the upstream scope and implementation before deciding whether to submit a new pull request. Thank you.