fix: read DASHSCOPE_API_KEY from environment in GovDoc2Poster basemodel

[Fahmid-Arman]

What type of PR is this?
/kind bug

What this PR does / why we need it:
The GovDoc2Poster example ignored the DASHSCOPE_API_KEY environment variable, defaulting to the hardcoded "your_api" string. This caused downstream LLM/VLM components to fail authentication silently.

This PR updates basemodel.py to:

• Read the API key from kwargs or fallback to os.environ.get("DASHSCOPE_API_KEY").
• Raise an explicit ValueError if the key is missing or set to the placeholder, enforcing a fail-fast mechanism.
• Remove unreachable legacy warning logs.

Which issue(s) this PR fixes:
Fixes #530

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Fahmid-Arman
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[gemini-code-assist]

Code Review

This pull request improves API key handling in the base model by retrieving the key from kwargs or falling back to the DASHSCOPE_API_KEY environment variable, raising a ValueError if it is missing. Feedback suggests using or chaining instead of kwargs.get default arguments to robustly handle cases where api_key is explicitly passed as None.

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[Fahmid-Arman]

Feedback addressed:

• Refactored API key retrieval to use or chaining to properly handle cases where api_key is explicitly passed as None.

/assign @jaypume