🔒 [Security Fix] Remove hardcoded tokens from auth credential docstrings

[shivan4030]

🎯 What: Replaced hardcoded dummy tokens/credentials in docstring examples in src/google/adk/auth/auth_credential.py with descriptive placeholders (e.g., YOUR_TOKEN, YOUR_API_KEY, YOUR_USERNAME, YOUR_PASSWORD, YOUR_CLIENT_ID, YOUR_CLIENT_SECRET, YOUR_PROJECT_ID, YOUR_RESOURCE_ID).
⚠️ Risk: Security scanners flagged these dummy values as potential hardcoded secrets, creating false positives and alert noise.
🛡️ Solution: Used explicit descriptive placeholders instead of dummy values to prevent security scanners from incorrectly identifying them as vulnerabilities.

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PR created automatically by Jules for task 605596124533806991 started by @shivan4030

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