fix(ai): remove manual Accept-Encoding header so Mistral gzip responses decode

[martinconstam]

Problem

Any LLM task (LLM_TEXT_COMPLETE, LLM_CHAT_COMPLETE, …) using the Mistral provider always fails with:

Error while extracting response for type [org.springframework.ai.mistralai.api.MistralAiApi$ChatCompletion]
and content type [application/json] — caused by: Illegal character ((CTRL-CHAR, code 31)):
only regular white space (\r, \n, \t) is allowed between tokens

code 31 is 0x1F — the first byte of the gzip magic number (0x1F 0x8B). Jackson is being handed a gzip-compressed body that was never decompressed.

Root cause

MistralAI#createMistralAiApi builds the RestClient on an OkHttp3ClientHttpRequestFactory and additionally sets the Accept-Encoding header by hand:

RestClient.builder()
    .requestFactory(factory)
    .defaultHeader("Accept-Encoding", "gzip, deflate")

OkHttp only decompresses a gzip response transparently when it adds the Accept-Encoding header itself (via its BridgeInterceptor). Setting the header explicitly turns that automatic decompression off, so the gzipped body is passed straight to Jackson and parsing fails on the 0x1F byte.

The header was originally added to work around spring-ai#372, but it is unnecessary — and now actively harmful — since the client uses the OkHttp request factory, which handles content encoding on its own.

Fix

Remove the manual Accept-Encoding header and let OkHttp manage compression. Mistral responses then decode correctly.

Tests

Adds MistralAIGzipResponseTest, a regression test using MockWebServer that serves a gzip-encoded chat completion and asserts it is decoded correctly. It fails against the old code (gzip body reaches Jackson undecoded → code 31) and passes with this change. The test needs no API key, so it runs in CI — unlike the existing MistralAITest.IntegrationTests, which are gated on MISTRAL_API_KEY and therefore skipped in CI (which is why this regression went unnoticed).

Impact / scope

• Present since the initial AI module commit ("Support Agentic flows", Support Agentic flows #745); affects every release that ships the AI module (3.30.0 onward, including 3.31.0-rc.x).
• Mistral-provider specific; other providers are unaffected by this line.
• Verified against the live Mistral API and via the new test: responses parse once OkHttp manages the encoding.