fix: sync base_url and api_base for litellm multi-provider routing

[devin-ai-integration]

Summary

Fixes #5139. When LLM(base_url=...) is used without explicitly setting api_base, litellm never receives the custom endpoint because it reads api_base, not base_url. This causes requests to fall back to api.openai.com, breaking multi-provider setups (e.g. Scaleway + Nebius with different API keys and endpoints).

The fix adds a 4-line sync in LLM.__init__ so that whichever parameter the caller provides, both fields are populated:

• Only base_url provided → api_base is set to match
• Only api_base provided → base_url is set to match
• Both provided → both keep their explicit values
• Neither provided → both stay None

This ensures _prepare_completion_params always includes api_base in the kwargs passed to litellm.completion().

Review & Testing Checklist for Human

• Verify all internal litellm call sites go through _prepare_completion_params: This fix works because _prepare_completion_params passes api_base/api_key to litellm.completion(). If any code path (e.g. agent reasoning loops, tool selection) calls litellm without going through _prepare_completion_params, those calls would still lack the custom endpoint. Grep for direct litellm.completion / litellm.acompletion calls outside this path.
• End-to-end test with a real multi-provider crew: Create two LLM instances with different base_url/api_key values, assign them to different agents in the same crew, and run a task. Confirm each agent's requests go to the correct endpoint (e.g. via debug logging or network inspection).
• Check native provider paths are unaffected: The sync only runs in LLM.__init__ (the litellm fallback path). Native providers (OpenAI, Anthropic, Gemini, etc.) use their own __init__ via BaseLLM. Verify no regressions for LLM(model="gpt-4o") or LLM(model="anthropic/claude-3-sonnet").

Notes

• The issue also suggests a monkey-patch approach for litellm.completion/litellm.acompletion. This PR intentionally does not implement that — the simpler param-syncing fix should be sufficient since _prepare_completion_params already passes api_base/api_key to every litellm call.
• 9 new unit tests added covering all sync scenarios including _prepare_completion_params, multi-provider independence, litellm.completion mock verification, and copy/deepcopy preservation.
• tests (3.13) CI job was cancelled (infrastructure, not a failure); all other required checks (lint, type-checker, tests on 3.10/3.11/3.12) passed.

Link to Devin session: https://app.devin.ai/sessions/3ccb7f0ac5ba4e0c95dc59b9556576a3

[devin-ai-integration]

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[devin-ai-integration]

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