fix(sdc-agents-demo): use env vars for base_url and api_key

Addresses Gemini bot review on PR google#106:
- base_url is now overridable via SDC_BASE_URL with default
  https://sdcstudio.axius-sdc.com (the production endpoint).
- api_key is now read from SDC_API_KEY via os.environ.get rather than
  the literal "${SDC_API_KEY}" string, which was not substituted by
  pydantic and broke the demo as written.
- Docstring prerequisites updated to mention SDC_BASE_URL.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: twcook

contributing/samples/sdc_agents_demo/agent.py

@@ -21,11 +21,14 @@
 Prerequisites:
     pip install google-adk-community[sdc-agents]
     export SDC_API_KEY="your-sdcstudio-api-key"
+    export SDC_BASE_URL="https://sdcstudio.axius-sdc.com"  # optional; this is the default
 
 Usage:
     adk run .
 """
 
+import os
+
 from google.adk.agents import LlmAgent
 
 from google.adk_community.sdc_agents import (
@@ -39,8 +42,8 @@
 
 config = SDCAgentsConfig(
     sdcstudio={
-        "base_url": "https://sdcstudio.com",
-        "api_key": "${SDC_API_KEY}",
+        "base_url": os.environ.get("SDC_BASE_URL", "https://sdcstudio.axius-sdc.com"),
+        "api_key": os.environ.get("SDC_API_KEY", ""),
     },
     datasources={
         "sample": {