fix: remove unreachable null checks flagged by Sonar (S2583)

[azanux]

Remove unreachable null checks flagged by Sonar (S2583)

Summary

• SonarCloud flagged 3 dead-code null checks under rule java:S2583 ("condition always evaluates to false") on the new-code leak period.
• ChatModelObservationFilter.buildInputMessages / extractPrompt: chatContext.getRequest() is guaranteed non-null by Spring AI's ChatModelObservationContext contract, so the if (request == null) guards were unreachable.
• ObservationUtils.formatMessages: Message.role is a non-nullable Kotlin val, so the m.getRole() == null check was unreachable.
• No behavior change — these were defensive checks against conditions that can't occur given the current contracts.

Test plan

• mvn -pl embabel-agent-observability -am compile -DskipTests
• mvn -pl embabel-agent-observability test -Dtest=ChatModelObservationFilterTest,ObservationUtilsTest

[igordayen]

all good, except UnfoldingToolInjectionStrategy, just extra line?

[azanux]

For the space, I don’t think it was done on purpose. Maybe I was trying to remove something in another file, and the cursor happened to be focused on this file.
Will see if I can revert

[azanux]

@igordayen revert done