feat(react-agent-gemini): add Gemini entry and GeminiSpec

Gemini.of(modelName) returns GeminiSpec exposing a thinking sub-builder
that supports both Gemini 2.5's level() and the legacy budget() form.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

liteflow-react-agent/liteflow-react-agent-gemini/src/main/java/com/yomahub/liteflow/agent/gemini/Gemini.java

@@ -0,0 +1,8 @@
+package com.yomahub.liteflow.agent.gemini;
+
+public final class Gemini {
+    private Gemini() {}
+    public static GeminiSpec of(String modelName) {
+        return new GeminiSpec(modelName);
+    }
+}

liteflow-react-agent/liteflow-react-agent-gemini/src/main/java/com/yomahub/liteflow/agent/gemini/GeminiSpec.java

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+package com.yomahub.liteflow.agent.gemini;
+
+import com.yomahub.liteflow.agent.model.CredentialResolver;
+import com.yomahub.liteflow.agent.model.ModelSpec;
+import com.yomahub.liteflow.property.agent.AgentConfig;
+import com.yomahub.liteflow.property.agent.PlatformCredential;
+import io.agentscope.core.model.GeminiChatModel;
+import io.agentscope.core.model.GenerateOptions;
+import io.agentscope.core.model.Model;
+
+import java.util.function.Consumer;
+
+public class GeminiSpec extends ModelSpec<GeminiSpec> {
+
+    private final String modelName;
+    private String  thinkingLevel;
+    private Integer thinkingBudget;
+
+    public GeminiSpec(String modelName) { this.modelName = modelName; }
+
+    public GeminiSpec thinking(Consumer<GeminiThinking> c) {
+        GeminiThinking t = new GeminiThinking();
+        c.accept(t);
+        this.thinkingLevel  = t.getLevel();
+        this.thinkingBudget = t.getBudget();
+        return this;
+    }
+
+    public String  getModelName()      { return modelName; }
+    public String  getThinkingLevel()  { return thinkingLevel; }
+    public Integer getThinkingBudget() { return thinkingBudget; }
+
+    @Override
+    public Model resolve(AgentConfig cfg) {
+        PlatformCredential cred = CredentialResolver.requireFirstClass(
+                cfg.getGemini(), "liteflow.agent.gemini");
+
+        GeminiChatModel.Builder builder = GeminiChatModel.builder()
+                .apiKey(cred.getApiKey())
+                .modelName(modelName);
+        GenerateOptions options = buildGenerateOptions();
+        if (options != null) {
+            builder.defaultOptions(options);
+        }
+        if (getStream() != null) {
+            builder.streamEnabled(getStream());
+        }
+        return builder.build();
+    }
+
+    private GenerateOptions buildGenerateOptions() {
+        if (getTemperature() == null && getTopP() == null && getTopK() == null
+                && getMaxTokens() == null && getSeed() == null
+                && thinkingLevel == null && thinkingBudget == null) {
+            return null;
+        }
+        GenerateOptions.Builder b = GenerateOptions.builder();
+        if (getTemperature() != null)   b.temperature(getTemperature());
+        if (getTopP() != null)          b.topP(getTopP());
+        if (getTopK() != null)          b.topK(getTopK());
+        if (getMaxTokens() != null)     b.maxTokens(getMaxTokens());
+        if (getSeed() != null)          b.seed(getSeed());
+        if (thinkingLevel != null)      b.reasoningEffort(thinkingLevel);
+        if (thinkingBudget != null)     b.thinkingBudget(thinkingBudget);
+        return b.build();
+    }
+}

liteflow-react-agent/liteflow-react-agent-gemini/src/main/java/com/yomahub/liteflow/agent/gemini/GeminiThinking.java

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+package com.yomahub.liteflow.agent.gemini;
+
+/**
+ * Gemini 平台 thinking 子构建器。
+ * Gemini 2.5 使用 "thinking_level"（low/medium/high），老接口用 "thinking_budget"（token 数）。
+ */
+public final class GeminiThinking {
+    private String level;
+    private Integer budget;
+
+    public GeminiThinking level(String level)   { this.level = level;   return this; }
+    public GeminiThinking budget(int tokens)    { this.budget = tokens; return this; }
+
+    public String  getLevel()  { return level; }
+    public Integer getBudget() { return budget; }
+}

liteflow-testcase-el/liteflow-testcase-el-react-agent/src/test/java/com/yomahub/liteflow/agent/gemini/GeminiEntryTest.java

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+package com.yomahub.liteflow.agent.gemini;
+
+import com.yomahub.liteflow.agent.exception.AgentConfigException;
+import com.yomahub.liteflow.property.agent.AgentConfig;
+import io.agentscope.core.model.GeminiChatModel;
+import io.agentscope.core.model.Model;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class GeminiEntryTest {
+
+    @Test
+    void buildsGeminiChatModel() {
+        AgentConfig cfg = new AgentConfig();
+        cfg.getGemini().setApiKey("g-key");
+
+        Model model = Gemini.of("gemini-2.5-pro")
+                .temperature(0.6)
+                .thinking(t -> t.level("high"))
+                .resolve(cfg);
+
+        assertTrue(model instanceof GeminiChatModel);
+        assertEquals("gemini-2.5-pro", ((GeminiChatModel) model).getModelName());
+    }
+
+    @Test
+    void thinkingLevelAndBudgetStored() {
+        GeminiSpec spec = Gemini.of("gemini-2.5-pro")
+                .thinking(t -> t.level("medium").budget(1024));
+        assertEquals("medium", spec.getThinkingLevel());
+        assertEquals(1024, spec.getThinkingBudget());
+    }
+
+    @Test
+    void throwsWhenApiKeyMissing() {
+        AgentConfig cfg = new AgentConfig();
+        AgentConfigException ex = assertThrows(AgentConfigException.class,
+                () -> Gemini.of("gemini-2.5-pro").resolve(cfg));
+        assertTrue(ex.getMessage().contains("liteflow.agent.gemini.api-key"));
+    }
+}