fix(pipeline): treat gemini tool_calls+empty-args as truncation retry

Gemini returns finish_reason="tool_calls" (not "length") even when the
thinking budget exhausted max_tokens before tool arguments could be
emitted. Extend the existing truncation retry condition to detect this
case via an allowlist of mutating tools (write_file, edit, exec,
create_image, read_file) that virtually never legitimately call with
empty args. Nullary tools (datetime, heartbeat) pass through unchanged.

Trace: 019d8f33-2de1-7ab2-9a32-9df92cd610dd

internal/pipeline/think_stage.go

@@ -69,9 +69,14 @@ func (s *ThinkStage) Execute(ctx context.Context, state *RunState) error {
 		state.Think.TotalUsage.ThinkingTokens += resp.Usage.ThinkingTokens
 	}
 
-	// 6. Handle truncation: only retry when tool call args are truncated or malformed.
+	// 6. Handle truncation: retry when tool call args are truncated or malformed.
+	// Gemini returns finish_reason="tool_calls" (not "length") even when the thinking
+	// budget exhausted max_tokens before args could be emitted — detect via empty
+	// args on allowlisted mutating tools. Nullary tools (datetime, heartbeat) skip
+	// the heuristic so their legitimate empty-args calls pass through.
 	// Text-only truncation (no tool calls) is a valid long answer — deliver it.
-	truncated := resp.FinishReason == "length" && len(resp.ToolCalls) > 0
+	truncated := len(resp.ToolCalls) > 0 && (resp.FinishReason == "length" ||
+		(resp.FinishReason == "tool_calls" && toolCallsHaveMissingRequiredArgs(resp.ToolCalls)))
 	parseErr := !truncated && toolCallsHaveParseErrors(resp.ToolCalls)
 	if truncated || parseErr {
 		state.Think.TruncRetries++
@@ -157,3 +162,33 @@ func toolCallsHaveParseErrors(calls []providers.ToolCall) bool {
 	}
 	return false
 }
+
+// mutatingToolsRequireArgs is the static allowlist of tools where empty
+// arguments are virtually never legitimate. Production telemetry (30d) shows
+// 1/211 tool_call spans had empty args (the Gemini-3 budget-exhaustion trace);
+// datetime/heartbeat/web_search always carry args. Conservative scope — expand
+// only with telemetry justification.
+var mutatingToolsRequireArgs = map[string]struct{}{
+	"write_file":   {},
+	"edit":         {},
+	"exec":         {},
+	"create_image": {},
+	"read_file":    {},
+}
+
+// toolCallsHaveMissingRequiredArgs returns true when any call in the batch
+// targets a mutating tool from the allowlist but carries empty Arguments.
+// This is the Gemini-3 truncation signal: finish_reason="tool_calls" with
+// len(args)==0 on a tool we know requires params means the budget ran out
+// before args could be emitted.
+func toolCallsHaveMissingRequiredArgs(calls []providers.ToolCall) bool {
+	for _, tc := range calls {
+		if _, requires := mutatingToolsRequireArgs[tc.Name]; !requires {
+			continue
+		}
+		if len(tc.Arguments) == 0 {
+			return true
+		}
+	}
+	return false
+}