Release langchain-core==0.2.6

Changes since langchain-core==0.2.5

core: release 0.2.6 (#22868)
core[patch]: Treat type as a special field when merging lists (#22750)
core: In astream_events v2 propagate cancel/break to the inner astream call (#22865)
core[patch]: allowing latest packaging versions (#22792)
ci: add testing with Python 3.12 (#22813)
core[patch]: Update remaining root_validators (#22829)
core[patch]: update some root_validators (#22787)
core: fix mustache falsy cases (#22747)
core[patch]: Add missing type annotations (#22756)
core: add error message for non-structured llm to StructuredPrompt (#22684)
[Core] Unified Enable/Disable Tracing (#22576)