Fix unused variable, missing defaults, and silent misconfiguration in hotel booking sample

[Copilot]

openai_api_key was read from the environment but never used, causing silent misconfiguration. GITHUB_ENDPOINT had no fallback (passing None to the client), and the model ID was hardcoded rather than environment-driven.

Changes

• Fail fast on missing auth: removed unused openai_api_key; added explicit ValueError when GITHUB_TOKEN is unset
• GITHUB_ENDPOINT default: falls back to https://models.inference.ai.azure.com instead of None
• GITHUB_MODEL_ID env var: model ID is now read from the environment with "gpt-4o" as default
• Remove duplicate env lookup: reuse the already-assigned github_token local variable in OpenAIChatClient
• Comment accuracy: updated # GitHub Models or Azure OpenAI → # GitHub Models

# Before
openai_api_key = os.getenv("OPENAI_API_KEY")          # unused
github_token = os.getenv("GITHUB_TOKEN")
chat_client = OpenAIChatClient(
    base_url=os.environ.get("GITHUB_ENDPOINT"),        # can be None
    api_key=os.environ.get("GITHUB_TOKEN"),            # duplicate lookup
    model_id="gpt-4o",                                 # hardcoded
)

# After
github_token = os.getenv("GITHUB_TOKEN")
if not github_token:
    raise ValueError("GITHUB_TOKEN environment variable is not set. Please set it to use GitHub Models.")
chat_client = OpenAIChatClient(
    base_url=os.environ.get("GITHUB_ENDPOINT", "https://models.inference.ai.azure.com"),
    api_key=github_token,
    model_id=os.environ.get("GITHUB_MODEL_ID", "gpt-4o"),
)

[github-actions]

👋 Thanks for contributing @copilot! We will review the pull request and get back to you soon.

[leestott]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in commit 6a36d0e. The conflict was in hotel_booking_workflow_sample.py where origin/main had merged a MiniMax provider addition (PR #474). The resolved version uses origin/main's multi-provider implementation, which also includes all the original review fixes (GITHUB_ENDPOINT default, GITHUB_MODEL_ID env var, proper provider selection).

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.