`--json` session output and sessions.json leak stdio `env` values in plaintext

[jirispilka]

Refs apify/apify-mcp-server#1162.

mcpc --json @session and sessions.json leak stdio env values in plaintext (headers are redacted, env is not).

Config:

"env": { "SECRET_TOKEN": "${FAKE_SECRET_TOKEN}" }

mcpc --json @probe | jq '._mcpc.server.env':

{ "SECRET_TOKEN": "sk_this_is_a_totally_fake_secret_12345" }

Same plaintext value also stored in sessions.json on disk.