release(langgraph): 1.2.8 (#8292)

## Summary

Releases `langgraph` 1.2.8.

Bumps the package version `1.2.7` -> `1.2.8` and propagates it into the
`langgraph`, `prebuilt`, and `sdk-py` lockfiles. No dependency floor or
source changes.

## Changes

- Update `libs/langgraph/pyproject.toml` to version `1.2.8`.
- Update the editable `langgraph` package entries in
`libs/langgraph/uv.lock`, `libs/prebuilt/uv.lock`, and
`libs/sdk-py/uv.lock`.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

libs/langgraph/pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "langgraph"
-version = "1.2.7"
+version = "1.2.8"
 description = "Building stateful, multi-actor applications with LLMs"
 authors = []
 requires-python = ">=3.10"

libs/langgraph/uv.lock

@@ -1438,7 +1438,7 @@ wheels = [
 
 [[package]]
 name = "langgraph"
-version = "1.2.7"
+version = "1.2.8"
 source = { editable = "." }
 dependencies = [
     { name = "langchain-core" },

libs/prebuilt/uv.lock

@@ -285,7 +285,7 @@ wheels = [
 
 [[package]]
 name = "langgraph"
-version = "1.2.7"
+version = "1.2.8"
 source = { editable = "../langgraph" }
 dependencies = [
     { name = "langchain-core" },

libs/sdk-py/uv.lock

@@ -298,7 +298,7 @@ wheels = [
 
 [[package]]
 name = "langgraph"
-version = "1.2.7"
+version = "1.2.8"
 source = { editable = "../langgraph" }
 dependencies = [
     { name = "langchain-core" },