Content block start text preserved; ValueError messages added

Changes since langchain-anthropic==1.4.7

release(anthropic): 1.4.8 (#38490) fix(anthropic): keep initial text on content_block_start (#38442) chore: bump langgraph-checkpoint from 4.1.0 to 4.1.1 in /libs/partners/anthropic (#38479) fix(core): add messages to bare raise ValueError calls (#38158)