Nested subgraphs inherit parent checkpoint; stream abort fixed

Changes since 1.2.5

• release(langgraph): 1.2.6 (#8139)
• fix: nested subgraph inherits parent checkpoint_ns (regression in 1.2.3) (#8053)
• fix: cancel running subgraphs on v3 stream abort [closes #8029] (#8057)
• release(cli): 0.4.30 (#8101)
• docs: standardize package README.md structure (#8064)
• chore(deps): bump tornado from 6.5.5 to 6.5.6 in /libs/langgraph (#8063)