Add an OpenAI sample to spring-agent-flow-samples

[ahmed-sekka]

spring-agent-flow-samples only demonstrates the framework with Mistral. A parallel OpenAI sample shows that the framework is provider-agnostic and broadens the audience.

Acceptance criteria

• spring-ai-starter-model-openai added as a sample-scoped dependency
• OpenAiCoordination.java mirroring the structure of MultiAgentCoordination.java
• Runnable via mvn -pl spring-agent-flow-samples exec:java -Dexec.mainClass=...
• README sample table updated with the new entry

Notes

Use the OPENAI_API_KEY environment variable; do not hardcode keys.