Backport: [v6.0] fix(langchain): reset streamEvents text between model invocations

[github-actions]

This is an automated backport of #16789 to the release-v5.0 branch. FYI @gr2m
This backport has conflicts that need to be resolved manually.

git cherry-pick output

CONFLICT (modify/delete): packages/langchain/src/adapter.ts deleted in HEAD and modified in c8e38eb83c ([v6.0] fix(langchain): reset streamEvents text between model invocations (#16789)).  Version c8e38eb83c ([v6.0] fix(langchain): reset streamEvents text between model invocations (#16789)) of packages/langchain/src/adapter.ts left in tree.
error: could not apply c8e38eb83c... [v6.0] fix(langchain): reset streamEvents text between model invocations (#16789)
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"