add BurnCloud as a new Provider

[ka1i]

Description

• Added full BurnCloud platform integration: new config (BurnCloudConfig), backend (BurnCloudModel), factory wiring, enums, and utility updates.
• Exposed environment variables in .env.example, plus comprehensive Mintlify docs for config/model/enums.
• Created three runnable BurnCloud examples (basic chat, streaming CLI, FunctionTool usage) and a pytest covering the model to guard against regressions.

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTION guide (required)
• I have linked this PR to an issue using the Development section on the right sidebar or by adding Fixes #issue-number in the PR description (required)
• I have checked if any dependencies need to be added or updated in pyproject.toml and uv lock
• I have updated the tests accordingly (required for a bug fix or a new feature)
• I have updated the documentation if needed:
• I have added examples if this is a new feature

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[ka1i]

@Wendong-Fan Hi, could you please take a look at this?