feat(ai-gateway): Add how-to for LangGraph

[tomek-labuk]

Description

Fixes #issue

Preview Links

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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[gitguardian]

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🔎 Detected hardcoded secret in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
27580108	Triggered	Generic Private Key	3f6ecaf	app/mesh/meshexternalservice.md	View secret

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