feat(langchain): Gate prompt/response collection on data_collection option (#7173)

Modify the Langchain integration to respect the data_collection config
for controlling whether prompts, responses, tool inputs/outputs, and
embeddings inputs are captured in spans.

When data collection is enabled, the new gen_ai.inputs and
gen_ai.outputs flags control what data is collected. When data
collection is not configured, falls back to legacy send_default_pii and
include_prompts settings for compatibility.

Available tools keep their previous behaviour when data collection is
not configured: they were recorded unconditionally before, so they
remain unaffected by send_default_pii there.

Refs PY-2588
Refs #6748

Author: ericapisani