ChatClient advisors NPE when context contains null values

Bug description

ChatClientRequest only forbids null keys, not null values. The advisors (ChatModelStreamAdvisor, ChatModelCallAdvisor, SafeGuardAdvisor) call Map.copyOf(request.context()), and that blows up with NullPointerException if any value is null. The call/stream fails before the model is invoked.

Steps to reproduce

1. Build a ChatClientRequest with a null context value, e.g. context("tenantId", null).
2. Use a ChatClient with default advisors (or directly use ChatModelStreamAdvisor/ChatModelCallAdvisor).
3. Call .stream(...) or .call(...).
4. Map.copyOf throws NPE inside the advisor.

Expected behavior

Either reject the request up front with a clear validation error, or avoid throwing NPE in the advisors when context contains null values.

Minimal complete reproducible example

class ChatClientRequestNullContextTests {

    @Test
    void nullContextValueTriggersNpeInAdvisor() {
        ChatClientRequest request = ChatClientRequest.builder()
            .prompt(new Prompt("hi"))
            .context("tenantId", null) // null value
            .build();

        // Same failure point as in the advisors:
        assertThatThrownBy(() -> Map.copyOf(request.context()))
            .isInstanceOf(NullPointerException.class);
    }
}

Affected code

• spring-ai-client-chat/src/main/java/org/springframework/ai/chat/client/advisor/ChatModelStreamAdvisor.java
• spring-ai-client-chat/src/main/java/org/springframework/ai/chat/client/advisor/ChatModelCallAdvisor.java
• spring-ai-client-chat/src/main/java/org/springframework/ai/chat/client/advisor/SafeGuardAdvisor.java
• spring-ai-client-chat/src/main/java/org/springframework/ai/chat/client/ChatClientRequest.java

Proposed resolution

Tighten ChatClientRequest validation to disallow null values in context, so invalid requests fail fast with a clear message, and the advisors can keep their immutable Map.copyOf calls.

Start with ChatClientRequest.java and inspect how context entries are validated, then review the Map.copyOf call sites in ChatModelStreamAdvisor.java, ChatModelCallAdvisor.java, and SafeGuardAdvisor.java. Add or update a test based on ChatClientRequestNullContextTests to verify that a null context value fails with a clear validation error before advisor execution.