docs: remove old AI Gateway LangGraph lab (DOC-2205)

[micheleRP]

What

Removes the obsolete "Build a LangGraph Agent with the Redpanda AI Gateway" lab. The AI Gateway has been rewritten, so this lab is no longer accurate.

Deletes:

• ai-agents/langchain-agent/ — the lab source (README + Python agent)
• docs/modules/ai-agents/ — the now-empty docs module (held only the symlinked langchain-agent.adoc page)

Notes

• No nav.adoc or antora.yml changes needed (labs nav is auto-generated; the module wasn't explicitly registered).
• No inbound xrefs/links to this lab exist in any docs repo, so no broken links result.
• A companion docs-site PR adds 301 redirects for the published URLs to the /labs/ landing page.

Jira: https://redpandadata.atlassian.net/browse/DOC-2205

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[Feediver1]

Docs standards review (lab removal)

Files reviewed: 1 .adoc page (the deleted symlink), plus 13 supporting files (Python source, configs, lock files)
Net change: 0+/3478- — pure deletion.

What this PR does

Removes the obsolete LangGraph AI Gateway lab from redpanda-labs:

• ai-agents/langchain-agent/ directory (lab source: README.adoc, Python agent, poetry lockfiles, configs)
• docs/modules/ai-agents/pages/langchain-agent.adoc (the docs symlink that surfaced the README in the Antora-built site)

Rationale per the PR body: the AI Gateway has been rewritten, so the lab is no longer accurate.

Cross-repo coordination

Companion redirect PR is open and clean:

• redpanda-data/docs-site#179 adds two 301 redirects to netlify.toml (/redpanda-labs/ai-agents/langchain-agent/ and /labs/ai-agents/langchain-agent/ → /labs/). 12+/0-, all CI green. The PR body explicitly notes the ordering: the specific redirect is placed before the /redpanda-labs/* catch-all so it isn't rewritten to the now-dead canonical path. Worth merging together with this one to avoid even a brief window where the URLs 404.

Critical issues

None.

Verification of the PR body's claims

Claim	Status
"No nav.adoc or antora.yml changes needed (labs nav is auto-generated)"	✅ Verified. docs/antora.yml doesn't enumerate modules; Antora auto-discovers from docs/modules/.
"No inbound xrefs/links to this lab exist in any docs repo"	✅ Verified across docs, cloud-docs, rp-connect-docs, docs-site source trees. The matches in docs-site/docs/ are auto-generated build artifacts (sitemap, llms-full.txt, the built HTML/MD) that regenerate on the next Antora build.
Companion redirect PR exists	✅ Confirmed (#179, with proper ordering).
Module directory fully cleaned	✅ docs/modules/ai-agents/ returns 404 on the PR branch. ai-agents/ top-level also fully removed.
No leftover CI / workflow references	✅ build-docs.yml, test-docs.yml, update-deps.yml have zero matches for langchain-agent, LangGraph, or ai-agent. Makefile and package.json clean too.

Suggestions

1. Consider redirecting to a successor lab if one exists. The redirect lands users on /labs/ (the generic labs index). If the AI Gateway rewrite has a new lab or example that's the replacement for the LangGraph one, the redirect target could be that page instead — better UX for someone who arrived from a Google result or stale blog link expecting the LangGraph example specifically. If no successor exists yet, /labs/ is the right fallback.

2. Worth a one-line note in either the AI Gateway docs or wherever the rewrite is documented saying "The previous LangGraph example was removed and is being updated for the new Gateway" (or similar). Otherwise the redirect is the only signal customers get that this content existed and was replaced — and they only hit it if they go to the old URL. A pointer in the AI Gateway product docs would catch customers who were looking for an example for the first time and don't have the old URL.

3. Minor nit on the PR title. "LangGraph lab" vs. the directory name langchain-agent — LangGraph is part of the LangChain ecosystem so both names are technically defensible, but the inconsistency between the human-readable PR title and the directory name might confuse archaeologists in six months. Not blocking; just an observation.

Impact on other files

• redpanda-data/docs-site#179 is the impact-on-other-files response, and it's already open with the correct redirect ordering. Should land together with this PR.
• No What's New / release-notes entry. Labs aren't typically tracked in product release notes, so this is probably fine. If there's a labs-specific changelog or AI Gateway "what's changing" page, a one-line entry would help discoverability.
• No broken inbound links. Confirmed in source trees of all four sibling docs repos.

What works well

• Cross-repo coordination done upfront. The companion redirect PR (#179) was opened one minute after this one, with the labs URL pattern and the catch-all-ordering concern correctly handled.
• Complete cleanup. No half-removed module directories, no stale workflow references, no Makefile/package.json mentions of the deleted lab. The PR description's verification claims are all accurate.
• Symlink handling is correct. The deleted .adoc is a symlink (deleted file mode 120000) — removing the symlink and the README it pointed at is the right pattern for Antora-on-labs.
• The "before the catch-all" ordering call-out in #179 shows the author thought about exactly the kind of shadowing bug that bit (or appeared to bite) the earlier redirects PR. That's deliberate, careful work.
• Clean diff. 0+/3478- — pure deletion, easy to verify completeness.