Add getting-started module: ChatClient fluent API, system/user prompts, streaming, and provider switching via spring.ai.model.chat

getting-started/output/01-plain-call.txt

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+# GET /api/chat -- ChatClient.prompt().user(...).call().content()
+
+$ curl 'http://localhost:34201/api/chat?message=What+package+is+ChatClient+in?'
+
+You said: "What package is ChatClient in?". That is 30 characters.

getting-started/output/02-system-template.txt

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+# GET /api/chat/as -- a templated system prompt, filled via .param("voice", ...)
+
+$ curl 'http://localhost:34201/api/chat/as?voice=pirate&message=Where+is+my+jar+cached'
+
+Response body:
+As a pirate: Where is my jar cached -- yes, and it is exactly 22 characters long.
+
+Exact text FakeChatModel received (proves the {voice} placeholder was substituted by ChatClient before the ChatModel ever saw the prompt):
+Answer in the voice of a pirate, still in two sentences.Where is my jar cached

getting-started/output/03-streaming.txt

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+# GET /api/chat/stream -- ChatClient.prompt().user(...).stream().content()
+
+$ curl -N 'http://localhost:34201/api/chat/stream?message=Stream+this'
+
+Chunks received: 1
+Joined: You said: "Stream this". That is 11 characters.

getting-started/output/04-provider-switch.txt

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+# Which ChatModel class spring.ai.model.chat selects
+
+spring.ai.model.chat=openai  ->  org.springframework.ai.openai.OpenAiChatModel
+spring.ai.model.chat=ollama  ->  org.springframework.ai.ollama.OllamaChatModel

getting-started/output/05-real-network-round-trip.txt

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+# A real HTTP round trip to OpenAI (not scripted) -- captured manually, not by run-all.sh
+
+This one is not reproducible by scripts/run-all.sh: it needs a live network path to
+api.openai.com, which the automated test suite deliberately does not depend on (see
+ChatClientEndpointsTest, which uses FakeChatModel instead). It is included because it is the
+one exhibit in this module that touches a real provider, and it proves the request pipeline
+-- ChatClient, the OpenAiChatModel bean spring.ai.model.chat=openai selected, and the OpenAI
+Java SDK underneath it -- is wired correctly all the way to OpenAI's own server, not just to
+a stub.
+
+Command (full application, spring-boot:run, a syntactically valid but fake key):
+
+  $ export OPENAI_API_KEY=sk-test-placeholder-not-a-real-key
+  $ curl 'http://localhost:8080/api/chat?message=hi'
+
+Response: HTTP 500, body in 05-real-openai-401-response-body.txt
+
+Application log root cause (grepped from the running process's stdout):
+
+  com.openai.errors.UnauthorizedException: 401: Incorrect API key provided: sk-test-**********************-key. You can find your API key at https://platform.openai.com/account/api-keys.
+
+That 401 comes from OpenAI's own server, not from Spring AI or from this sandbox -- OpenAI
+validated the key format, rejected it, and said so in its own error class
+(com.openai.errors.UnauthorizedException). A malformed request, a wrong base URL, or a client
+that never left the JVM would not look like this. Swap in a real key and this becomes a real
+reply.

getting-started/output/05-real-openai-401-response-body.txt

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+{"timestamp":"2026-09-23T03:53:37.888Z","status":500,"error":"Internal Server Error","path":"/api/chat"}

getting-started/output/06-stream-not-supported-original-failure.txt

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+# The original surefire failure that led to overriding FakeChatModel.stream()
+
+Captured from `mvn -B -o test` before FakeChatModel had a stream() override -- ChatModel's own
+default implementation was assumed (wrongly) to wrap call() in a single-element Flux. Trimmed to
+the relevant frames; jar version suffixes kept exactly as printed.
+
+2026-09-23T03:50:42.884Z ERROR 1145 --- [getting-started] [o-auto-1-exec-3] o.s.ai.chat.model.MessageAggregator      : Aggregation Error
+
+java.lang.UnsupportedOperationException: streaming is not supported
+	at org.springframework.ai.chat.model.ChatModel.stream(ChatModel.java:65) ~[spring-ai-model-2.0.1.jar:2.0.1]
+	at org.springframework.ai.chat.client.advisor.ChatModelStreamAdvisor.adviseStream(ChatModelStreamAdvisor.java:53) ~[spring-ai-client-chat-2.0.1.jar:2.0.1]
+	at org.springframework.ai.chat.client.advisor.DefaultAroundAdvisorChain.lambda$nextStream$5(DefaultAroundAdvisorChain.java:158) ~[spring-ai-client-chat-2.0.1.jar:2.0.1]