fix(core): parse stdout only in ProcessOutput.json()

[AbhiPra24]

Fixes #1505

Summary of Changes

• Updated ProcessOutput.prototype.json() in src/core.ts to parse this.stdout instead of this.stdall (combined stdout + stderr).
• Added unit test cases in test/core.test.js confirming that commands/ProcessOutput instances with non-JSON logs on stderr parse stdout JSON correctly without throwing parsing errors.

[goransh-walia]

Clean and minimal — appreciate that it stays scoped to the actual fix.