[Feature]: Add basic auth for GigaChat provider

[krakenalt]

Relevant issues

Fixes: #19339

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• My PR passes all unit tests on make test-unit
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CI (LiteLLM team)

CI status guideline:

• 50-55 passing tests: main is stable with minor issues.
• 45-49 passing tests: acceptable but needs attention
• <= 40 passing tests: unstable; be careful with your merges and assess the risk.

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Type

🧹 Refactoring
📖 Documentation
✅ Test

Changes

• Adds Basic Auth support for GigaChat token retrieval via /token using GIGACHAT_USER + GIGACHAT_PASSWORD (with tests).
• Keeps OAuth flow via GIGACHAT_CREDENTIALS / GIGACHAT_API_KEY, adds base64 credential validation.
• Introduces litellm/llms/gigachat/common_utils.py (constants, build_url(), exceptions, USER_AGENT).
• Updates chat transformation (more params, tool/function handling, structured output support, image upload hooks, etc.).
• Updates file upload path to use the shared URL builder + headers.
• Updates docs and tests accordingly.

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Project	Deployment	Review	Updated (UTC)
litellm	Ready	Preview, Comment	Jan 19, 2026 10:32am