Use native Pay tool in PCI collection sample

[anusha-telnyx]

Summary

• switch the PCI payment collection sample from the old assistant webhook workaround to the native Telnyx Pay (BETA) tool
• make moonshotai/Kimi-K2.6 the default assistant model
• simplify the Flask app to answer calls, start the assistant, receive Pay over Voice events, serve the mock processor, and show sanitized dashboard milestones
• update README/API/GUIDE/.env example and catalog wording for the native Pay tool flow

Validation

• python3 -m py_compile ai-pci-protected-payment-collection-python/app.py ai-pci-protected-payment-collection-python/provision_assistant.py
• Flask smoke test for /health and /webhooks/payment-processor
• ruby -e 'require "json"; require "yaml"; JSON.parse(File.read("catalog.json")); YAML.load_file("scripts/examples_mapping.yaml")'
• git diff --check