remove Spring AI Dependencies, replace by spring-ai-bom; add flatten-maven-plugin

[lambochen]

Remove duplicate Spring AI dependencies, as they are already managed by spring-ai-bom.

<!-- Spring AI BOM -->
            <dependency>
                <groupId>org.springframework.ai</groupId>
                <artifactId>spring-ai-bom</artifactId>
                <version>${spring-ai.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>

[markpollack]

Thank you for the contribution, @lambochen! Using spring-ai-bom for dependency management is a good idea.

The parent POM has been restructured significantly since this PR was opened (artifact renames, module reorganization in v0.11.0), so this PR would need a complete rewrite to apply cleanly. Closing as stale.

If you'd like to revisit this against the current codebase, we'd welcome a fresh PR!