fix(langchain): bump minimum langchain-core to 0.3.73 (#35608)

The within-batch deduplication fix (langchain-core 0.3.73, #32273)
changed `num_skipped` counting in the indexing API. Tests in
`libs/langchain` were updated to expect the new behavior, but the
minimum version was left at 0.3.72, causing [pre-release checks to
fail](https://github.com/langchain-ai/langchain/actions/runs/22783515938/job/66097466123).

Created with [Deep Agents
CLI](https://docs.langchain.com/oss/python/deepagents/cli/overview).