[bellCold]

Problem

ChatClient.mutate() loses some default settings.
It copies them only under conditions that are not related to them:

Setting	Copied only when
Advisor params	the client has at least one advisor
User params, media and metadata	the client has default user text
System params and metadata	the client has default system text

So if you set one of these defaults on its own, the mutated client does not have it.

Example

ChatClient client = ChatClient.builder(chatModel)
	.defaultAdvisors(AdvisorParams.ENABLE_NATIVE_STRUCTURED_OUTPUT)
	.defaultUser(u -> u.param("lang", "fr").metadata("k", "v"))
	.build();

ChatClient mutated = client.mutate().build();
// In "mutated", the advisor params, user params and user metadata are all empty.

Real effects:

• A client that enables native structured output with AdvisorParams.ENABLE_NATIVE_STRUCTURED_OUTPUT loses it after mutate().
• A client that turns off AdvisorParams.toolCallingAdvisorAutoRegister(false) gets the ToolCallingAdvisor registered again after mutate().

Cause

The advisor params were added to mutate() in #3459, inside the existing check for advisors.
User and system params were already inside the checks for user and system text.

Fix

mutate() now always copies advisor params, user and system params, media and metadata.
It still sets the user and system text only when there is text.

Tests

• The existing testMutate always set both advisors and text, so it did not catch this.
• The new testMutateKeepsParamsWithoutAdvisorsOrText fails without this change and passes with it.
• All unit tests in spring-ai-client-chat pass.