fix: Use the configured DeepSeek FIM base path

[mvanhorn]

Description

DeepSeek fill-in-the-middle autocomplete shows no suggestion and the extension logs a stream error. The same model and key return a normal streamed completion when DeepSeek’s beta completions endpoint is called directly. FIM requests always resolved the relative path beta/completions against the configured API base. A base whose path is already /beta/ therefore becomes /beta/beta/completions, and the completions call fails. A root base such as https://api.deepseek.com/ still resolves to /beta/completions, which is the path those bases need.

Both FIM call sites now build the endpoint with the same helper. When the pathname ends with /beta, trailing slashes are stripped and the request targets completions on that base. Otherwise the helper still resolves beta/completions, covering the default root and bases written without a trailing slash. The added tests assert the /beta/ URL and JSON body, the default root URL, the concatenated stream text, and that a failed response throws the response body.

Fixes #12600

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Tests

No local test run is recorded for the continue Deepseek change to core/llm/llms/Deepseek.ts and packages/openai-adapters/src/apis/DeepSeek.ts.