feat(llms): add transformRequestBody hook and improve prompt assembly (#480)

* feat(llms): add transformRequestBody hook and refine prompt handling

* docs(website): document transformRequestBody usage

* refactor(extension): keep function-valued config handling consistent in useAgent

* feat: simplify `transformRequestBody`

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Co-authored-by: Simon <10131203+gaomeng1900@users.noreply.github.com>

packages/llms/src/index.ts

@@ -21,6 +21,7 @@ export function parseLLMConfig(config: LLMConfig): Required<LLMConfig> {
 		apiKey: config.apiKey || '',
 		temperature: config.temperature ?? DEFAULT_TEMPERATURE,
 		maxRetries: config.maxRetries ?? LLM_MAX_RETRIES,
+		transformRequestBody: config.transformRequestBody ?? ((requestBody) => requestBody),
 		disableNamedToolChoice: config.disableNamedToolChoice ?? false,
 		customFetch: (config.customFetch ?? fetch).bind(globalThis), // fetch will be illegal unless bound
 	}