fix: add reasoning content field support in chat body

[uinstinct]

Description

Add reasoning_content field support. Deepseek requires reasoning support to be passed back when using tool calls.

closes #8989
resolves CON-5302

verified by changing ContinueProxy.ts

...
  constructor(options: LLMOptions) {
    super(options);
    this.configEnv = options.env;
    // This it set to `undefined` to handle the case where we are proxying requests to Azure. We pass the correct env vars
    // needed to do this in `extraBodyProperties` below, but if we don't set `apiType` to `undefined`, we end up proxying to
    // `/openai/deployments/` which is invalid since that URL construction happens on the proxy.
    this.apiType = undefined;
    this.actualApiBase = options.apiBase;
    this.apiKeyLocation = options.apiKeyLocation;
    this.envSecretLocations = options.envSecretLocations;
    this.orgScopeId = options.orgScopeId;
    this.onPremProxyUrl = options.onPremProxyUrl;
    if (this.onPremProxyUrl) {
      this.apiBase = new URL("model-proxy/v1/", this.onPremProxyUrl).toString();
    }

    // Set reasoning field support based on underlying provider
    const { provider } = parseProxyModelName(this.model);
    if (provider === "deepseek") {
      this.supportsReasoningContentField = true;
      this.supportsReasoningField = false;
    }
  }
...

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• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Summary by cubic

Adds reasoning_content support to chat requests and enables it for Deepseek so tool calls receive the model’s reasoning as required. Addresses Linear CON-5302 and fixes the Deepseek tool-call behavior reported in #8989.

• Bug Fixes
  • Added supportsReasoningContentField capability to BaseLLM and passed it through chat body creation.
  • Included reasoning_content when the previous message is role "thinking" in openaiTypeConverters, gated by provider support.
  • Configured Deepseek to use reasoning_content (supportsReasoningContentField=true, supportsReasoningField=false).

^{Written for commit ac93dd9. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

[RomneyDa]

consider else if, nbl

[uinstinct]

i think these if conditions are independent (includeReasoningContent can be present alongside includeReasoning)