feat(auth): support Kubernetes inline credentials

[MoonBoi9001]

Summary

Adds support for inline JSON credentials while maintaining official Google Cloud API usage.

Problem

The auth refactor (PR #42) removed inline credential support.

Solution

New function: prepare_credentials_for_adc()

• Detects credential format (inline JSON vs file path)
• For inline JSON: writes to /tmp/gcp-credentials.json with 0o600 permissions, updates env var
• For file paths: validates existence (backward compatible)
• Clears sensitive data from memory after use

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Release: Minor bump (0.4.2 → 0.4.3)

[MoonBoi9001]

Also fixed: Leak of sensitive data to slack channel via webhook notification caused by error messages being too broad and catching the full error message