ShellSession._collect_output times out when command output lacks a trailing newline

### Submission checklist - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs ShellSession._collect_output uses readline() to read stdout and detects completion with data.startswith(marker). When a command's last output line has no trailing \n (e.g. head, cat, grep on a minified single-line JSON file), readline() blocks until the next newline arrives — which only comes from the marker's own printf call. The result is that both are returned in a single readline() read: `{"key":"value"}__LC_SHELL_DONE__abc123 0\n` The startswith(marker) check fails, the loop receives nothing further, and the command times out after the full command_timeout (default 30 s). Expected behaviour The command completes immediately and returns `{"k":"v"}`. Actual behaviour Error: Command timed out after 30.0 seconds. Proposed fix Change line in _collect_output from: ``` if source == "stdout" and data.startswith(marker): _,…