Add spring-ai-google-genai module using Google's new GenAI SDK

Introduces a new module that provides an alternative to spring-ai-vertex-ai-gemini
using Google's latest GenAI SDK. This implementation offers the same functionality
and API surface as the existing Vertex AI module while leveraging Google's new
unified AI platform.

Key Features:
- GoogleGenAiChatModel with identical functionality to VertexAiGeminiChatModel
- Complete support for Gemini Pro, 1.5 Pro, and 2.0 Flash model variants
- Full tool/function calling capabilities via GoogleGenAiToolCallingManager
- Streaming support with reactive programming model
- Comprehensive safety settings and content filtering
- Multi-modal content support (text, images, PDFs)
- Google Search Retrieval integration
- Built-in retry mechanisms and Micrometer observability

Technical Implementation:
- Mirrors the architecture and patterns of spring-ai-vertex-ai-gemini
- Uses Google's com.google.genai.Client instead of Vertex AI SDK
- Maintains full API compatibility with existing Spring AI patterns
- Includes comprehensive test suite covering all functionality areas
- Proper Maven integration and AOT support

Test Coverage:
- 12 comprehensive test classes mirroring vertex-ai-gemini tests
- Integration tests for chat, streaming, tool calling, and observations
- Multi-modal content testing with images and PDFs
- Safety settings and retry mechanism validation
- Schema conversion and JSON handling tests

Signed-off-by: ddobrin <[email protected]>

Author: ddobrin

models/spring-ai-google-genai/README.md

@@ -0,0 +1,24 @@
+[VertexAI Gemini Chat](https://docs.spring.io/spring-ai/reference/api/chat/vertexai-gemini-chat.html)
+
+### Starter - WIP
+```xml
+<dependency>
+    <groupId>org.springframework.ai</groupId>
+    <artifactId>spring-ai-starter-model-spring-ai-google-genai</artifactId>
+</dependency>
+```
+
+### Manual config
+```xml
+<dependency>
+    <groupId>org.springframework.ai</groupId>
+    <artifactId>spring-ai-google-genai</artifactId>
+</dependency>
+```
+
+### Environment variables
+```shell
+export GOOGLE_GENAI_USE_VERTEXAI=true  
+export GOOGLE_CLOUD_PROJECT='your-project-id'  
+export GOOGLE_CLOUD_LOCATION='your-region'
+```

models/spring-ai-google-genai/pom.xml

@@ -0,0 +1,103 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Copyright 2023-2024 the original author or authors.
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~      https://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<parent>
+		<groupId>org.springframework.ai</groupId>
+		<artifactId>spring-ai-parent</artifactId>
+		<version>1.1.0-SNAPSHOT</version>
+		<relativePath>../../pom.xml</relativePath>
+	</parent>
+	<artifactId>spring-ai-google-genai</artifactId>
+	<packaging>jar</packaging>
+	<name>Spring AI Model - Google GenAI</name>
+	<description>Google GenAI Gemini models support</description>
+	<url>https://github.com/spring-projects/spring-ai</url>
+
+	<scm>
+		<url>https://github.com/spring-projects/spring-ai</url>
+		<connection>git://github.com/spring-projects/spring-ai.git</connection>
+		<developerConnection>[email protected]:spring-projects/spring-ai.git</developerConnection>
+	</scm>
+
+	<properties>
+	</properties>
+
+	<dependencies>
+
+		<dependency>
+			<groupId>com.google.genai</groupId>
+			<artifactId>google-genai</artifactId>
+			<version>1.8.0</version>
+		</dependency>
+
+		<dependency>
+			<groupId>com.github.victools</groupId>
+			<artifactId>jsonschema-generator</artifactId>
+			<version>${victools.version}</version>
+		</dependency>
+		<dependency>
+			<groupId>com.github.victools</groupId>
+			<artifactId>jsonschema-module-jackson</artifactId>
+			<version>${victools.version}</version>
+		</dependency>
+
+		<!-- production dependencies -->
+		<dependency>
+			<groupId>org.springframework.ai</groupId>
+			<artifactId>spring-ai-model</artifactId>
+			<version>${project.parent.version}</version>
+		</dependency>
+
+		<dependency>
+			<groupId>org.springframework.ai</groupId>
+			<artifactId>spring-ai-retry</artifactId>
+			<version>${project.parent.version}</version>
+		</dependency>
+
+
+		<!-- Spring Framework -->
+		<dependency>
+			<groupId>org.springframework</groupId>
+			<artifactId>spring-context-support</artifactId>
+		</dependency>
+
+		<dependency>
+			<groupId>org.slf4j</groupId>
+			<artifactId>slf4j-api</artifactId>
+		</dependency>
+
+		<!-- test dependencies -->
+		<dependency>
+			<groupId>org.springframework.ai</groupId>
+			<artifactId>spring-ai-test</artifactId>
+			<version>${project.version}</version>
+			<scope>test</scope>
+		</dependency>
+
+		<dependency>
+			<groupId>io.micrometer</groupId>
+			<artifactId>micrometer-observation-test</artifactId>
+			<scope>test</scope>
+		</dependency>
+
+	</dependencies>
+
+</project>