fix(ai): omit tools field instead of sending empty array (#3650)

DashScope / Aliyun Qwen (OpenAI-compatible) rejects `tools: []`
with HTTP 400 `"[] is too short - 'tools'"`. Five providers used
a truthy check (`if (context.tools)`) that treated an empty array
as "send tools", so `pi --no-tools` produced `tools: []` in the
request body. Matching the Google provider's pattern, we now
guard on `context.tools.length > 0`:

- openai-completions.ts
- openai-responses.ts
- openai-codex-responses.ts
- azure-openai-responses.ts
- anthropic.ts

The openai-completions fallback that emits `tools: []` when the
conversation has tool history (required by LiteLLM / Anthropic
proxies) is preserved via the existing `else if (hasToolHistory)`
branch.

closes #3649

Co-authored-by: 槐聚 <huaiju@zbyte-inc.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Mario Zechner <badlogicgames@gmail.com>

packages/ai/src/providers/anthropic.ts

@@ -877,7 +877,7 @@ function buildParams(
 		params.temperature = options.temperature;
 	}
 
-	if (context.tools) {
+	if (context.tools && context.tools.length > 0) {
 		params.tools = convertTools(
 			context.tools,
 			isOAuthToken,

packages/ai/src/providers/azure-openai-responses.ts

@@ -236,7 +236,7 @@ function buildParams(
 		params.temperature = options?.temperature;
 	}
 
-	if (context.tools) {
+	if (context.tools && context.tools.length > 0) {
 		params.tools = convertResponsesTools(context.tools);
 	}
 

packages/ai/src/providers/openai-codex-responses.ts

@@ -340,7 +340,7 @@ function buildRequestBody(
 		body.service_tier = options.serviceTier;
 	}
 
-	if (context.tools) {
+	if (context.tools && context.tools.length > 0) {
 		body.tools = convertResponsesTools(context.tools, { strict: null });
 	}
 

packages/ai/src/providers/openai-completions.ts

@@ -504,7 +504,7 @@ function buildParams(
 		params.temperature = options.temperature;
 	}
 
-	if (context.tools) {
+	if (context.tools && context.tools.length > 0) {
 		params.tools = convertTools(context.tools, compat);
 		if (compat.zaiToolStream) {
 			(params as any).tool_stream = true;

packages/ai/src/providers/openai-responses.ts

@@ -230,7 +230,7 @@ function buildParams(model: Model<"openai-responses">, context: Context, options
 		params.service_tier = options.serviceTier;
 	}
 
-	if (context.tools) {
+	if (context.tools && context.tools.length > 0) {
 		params.tools = convertResponsesTools(context.tools);
 	}