feat: add query_params option to ModelProviderInfo to support Azure

[bolinfest]

As discovered in #1365, the Azure provider needs to be able to specify api-version as a query param, so this PR introduces a generic query_params option to the model_providers config so that an Azure provider can be defined as follows:

[model_providers.azure]
name = "Azure"
base_url = "https://YOUR_PROJECT_NAME.openai.azure.com/openai"
env_key = "AZURE_OPENAI_API_KEY"
query_params = { api-version = "2025-04-01-preview" }

This PR also updates the docs with this example.

While here, we also update wire_api to default to "chat", as that is likely the common case for someone defining an external provider.

Fixes #1365.

[github-actions]

PR Summary

Adds optional query_params support in ModelProviderInfo and centralises endpoint construction via get_full_url, enabling providers like Azure that require query-string arguments. Refactors call sites, updates docs/examples, and expands tests accordingly.

Notes

• query_params: HashMap<String, String> (default None) appended to /chat/completions or /responses depending on wire_api.
• get_full_url removes scattered string concatenation in chat_completions.rs & client.rs.
• Docs now clarify default wire_api = "chat" and show Azure/Ollama examples.
• New unit tests cover TOML deserialisation with/without query_params.

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Review

Nice, tidy enhancement that makes provider configs more flexible and reduces URL-building duplication.

✅ Looks good overall; just a couple of nits:

• Consider using the url crate to percent-encode query values and avoid manual k=v joins.
• get_full_url trims neither trailing slashes nor ensures leading ones—worth double-checking edge cases.

Otherwise, great addition! 🎉

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