Stop sending prompt cache keys on Claude chat conversions (#2003)

Responses conversions still use promptCacheKey, but chat completions now stay a pure shape transform. This keeps Claude -> chat requests aligned with providers that do not understand the field and keeps stream checks consistent with production behavior.

Constraint: Issue #1919 requires removing prompt_cache_key from Claude -> OpenAI Chat requests
Rejected: Add a runtime toggle for chat injection | requested behavior is unconditional removal
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep promptCacheKey limited to Claude -> Responses conversions unless a provider-specific contract is proven
Tested: cargo test anthropic_to_openai
Tested: cargo test anthropic_to_responses_with_cache_key
Tested: cargo test transform_claude_request_for_api_format_responses
Not-tested: Full src-tauri test suite
Related: #1919

src-tauri/src/services/stream_check.rs

@@ -372,7 +372,7 @@ impl StreamCheckService {
             anthropic_to_responses(anthropic_body, Some(&provider.id), is_codex_oauth)
                 .map_err(|e| AppError::Message(format!("Failed to build test request: {e}")))?
         } else if is_openai_chat {
-            anthropic_to_openai(anthropic_body, Some(&provider.id))
+            anthropic_to_openai(anthropic_body)
                 .map_err(|e| AppError::Message(format!("Failed to build test request: {e}")))?
         } else {
             anthropic_body