Fix Advisor NPEs by enforcing non-null context in ChatClientRequest/Response

[Nhahan]

Fixes #4952

Approach

• Fail fast on bad input by disallowing null context keys/values in ChatClientRequest.
• Extend the same guardrails to ChatClientResponse for consistency, so direct response creation can’t sneak in nulls.
• Keep advisor code unchanged; they can continue to use Map.copyOf safely.

Scope

1. ChatClientRequest: constructor/builder enforce non-null keys and values (including single key/value adder).
2. ChatClientResponse: constructor/builder enforce non-null keys and values for symmetry.
3. Tests:
   • Requests: null keys/values throw clear IllegalArgumentException.
   • Responses: null keys/values throw clear IllegalArgumentException.
   • Valid inputs still succeed; immutability semantics remain.

Test plan

• ./mvnw -pl spring-ai-client-chat -DskipITs test (module scope).
• Unit tests in ChatClientRequestTests and ChatClientResponseTests cover null key/value rejection.

Backward compatibility

• This change enforces non-null values in the context map.
• Existing code that passes null values will now throw IllegalArgumentException immediately at construction time, rather than causing an NPE later in Advisors.