docs: add PaySafe payment firewall

[corbinallison]

Adds the tool integration page for langchain-paysafe (PyPI): scan x402 payments for safety before settlement, with a callback that auto-tags provenance so prompt-injection-triggered payments are caught.

Overview

This adds the documentation for my newly created x402 payment firewall service.

Type of change

New docs

Checklist

• [x ] I have read the contributing guidelines, including the language policy
• [x ] I have tested my changes locally using docs dev
• [x ] All code examples have been tested and work correctly
• [x ] I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

(Internal team members only / optional): Create a preview deployment as necessary using the Create Preview Branch workflow

Additional notes

[langchain-oss-automated-triage]

Thanks for contributing a new integration docs page, corbinallison!

We receive a large number of PRs and review them as quickly as we can. Please bear with us as we work through the queue.

If you have already tagged a maintainer on this PR, do not tag them again.

[github-actions]

Thanks for opening a docs PR, corbinallison! When it's ready for review, please add the relevant reviewers:

• @mdrxy (Python integrations)

[corbinallison]

Mason Daugherty (@mdrxy)