fix(model-connections): use OpenAI-compatible base URL verbatim and deprecate raw provider

[AnishSarkar22]

Description

• Use the OpenAI-compatible base URL exactly as entered across chat, model discovery, and connection verification; SurfSense no longer auto-appends /v1, matching how LiteLLM and the OpenAI SDK treat api_base.
• Remove the ensure_v1 helper entirely; to_litellm now only special-cases Anthropic (strips a trailing /v1 so LiteLLM's native handler doesn't produce /v1/v1/messages) and passes every other provider's base URL through verbatim.
• Apply verbatim handling to all OpenAI-compatible discovery, including OpenRouter and Requesty; their default base URLs already include /v1, so real-world behavior is unchanged, but the path is no longer forced.
• Query {base_url}/models exactly as entered for discovery and verification.
• Deprecate openai_compatible_raw: hidden from the provider catalog so it can no longer be added, while kept in the registry so pre-existing connections still resolve and work; the merged openai_compatible (now verbatim) covers the former raw use case.
• Simplify the connect form to a single OpenAI-compatible option with a hint that the base URL is used as-is (include /v1), dropping the separate raw-endpoint guidance and the endpoint preview.
• Update unit tests: drop the ensure_v1 assertions and add coverage that a bare host is not rewritten with /v1 and that custom paths (e.g. /api/v3) are preserved.
• Correct the LM Studio and other-local-servers docs to state the base URL is used verbatim (include /v1) and lead 404 troubleshooting with a check that the base URL contains /v1.
• No database migration or backfill required; the only affected case is pre-existing bare-host openai_compatible connections, which fail loudly and are fixed by adding /v1 to the URL.

Motivation and Context

FIX #

Screenshots

API Changes

• This PR includes API changes

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation
• Dependency/Build system
• Breaking change
• Other (specify):

Testing Performed

• Tested locally
• Manual/QA verification

Checklist

• Follows project coding standards and conventions
• Documentation updated as needed
• Dependencies updated as needed
• No lint/build errors or new warnings
• All relevant tests are passing

High-level PR Summary

This PR changes how OpenAI-compatible base URLs are handled throughout the system. Previously, the system would automatically append /v1 to base URLs; now the URL is used exactly as entered by the user. The openai_compatible_raw provider is deprecated (hidden from the catalog but kept for backward compatibility), as the main openai_compatible provider now handles both standard and custom paths. Special handling for Anthropic (stripping version suffixes) and fixed endpoints like OpenRouter/Requesty (which still append /v1) are preserved. The UI, documentation, and tests are updated to reflect that users must include /v1 in their base URLs when needed.

⏱️ Estimated Review Time: 5-15 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_backend/app/services/provider_registry.py
2	surfsense_backend/app/services/model_resolver.py
3	surfsense_backend/app/services/model_connection_service.py
4	surfsense_backend/tests/unit/services/test_model_connections.py
5	surfsense_backend/app/routes/model_connections_routes.py
6	surfsense_web/components/settings/model-connections/default-connect-form.tsx
7	surfsense_web/content/docs/local-models/lm-studio.mdx
8	surfsense_web/content/docs/local-models/other-local-servers.mdx

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