Backport: feat(langchain): emit source-url/source-document parts from citations

[github-actions]

This is an automated backport of #15757 to the release-v6.0 branch. FYI @dflynn15
This backport has conflicts that need to be resolved manually.

git cherry-pick output

Auto-merging packages/langchain/src/adapter.test.ts
Auto-merging packages/langchain/src/adapter.ts
Auto-merging packages/langchain/src/types.ts
Auto-merging packages/langchain/src/utils.test.ts
Auto-merging packages/langchain/src/utils.ts
CONFLICT (content): Merge conflict in packages/langchain/src/utils.ts
error: could not apply c231e4206... feat(langchain): emit source-url/source-document parts from citations (#15757)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"