chore(llmobs/langchain): stop using in-process vcr

I suspect that the in-process VCR is leading to flaky tests as:

- VCR patches all http connections including possibly those to the test agent; and
- I don't trust in-process VCR monkeypatching very much

Instead, we can continue to use VCR but out of process by proxying through the
test agent which now supports a VCR feature.

Author: Kyle-Verhoog