refactor(spring): extract Observation decorators into shared dapr-spring-boot-observation module

Eliminates the verbatim duplication between dapr-spring-boot-autoconfigure
(Spring Boot 3.x) and dapr-spring-boot-4-autoconfigure (Spring Boot 4.x):
both modules previously held 100%-identical copies of ObservationDaprClient
(~820 LOC) and ObservationDaprWorkflowClient (~400 LOC, including the W3C
traceparent/tracestate formatters, repeated 4 times overall).

Introduces a new Spring-agnostic module dapr-spring/dapr-spring-boot-observation
under package io.dapr.spring.observation.client holding a single canonical copy
of both decorators plus a package-private TraceContextFormat helper that
replaces the 4 duplicated formatter copies. The module has zero Spring imports
so it can be shared safely across Spring Boot 3.x and 4.x without API drift.

Autoconfigure modules now depend on dapr-spring-boot-observation (optional),
replacing their direct opentelemetry-api dependency. Consumers who skip
observation wiring still don't pull OTel onto the classpath.

OtelTracingClientInterceptor is now a private static final singleton on
ObservationDaprWorkflowClient instead of allocated per client — justified by
its existing javadoc which already stated it is stateless and shareable.

Tests for the decorators themselves moved to the new module (33 pass); the
Spring-wiring test DaprClientObservationAutoConfigurationTest stays in boot 3
with updated imports.

Net: -1104 lines.
Signed-off-by: Javier Aliaga <javier@diagrid.io>

Author: javier-aliaga

dapr-spring/dapr-spring-boot-4-autoconfigure/pom.xml

@@ -90,8 +90,8 @@
       <optional>true</optional>
     </dependency>
     <dependency>
-      <groupId>io.opentelemetry</groupId>
-      <artifactId>opentelemetry-api</artifactId>
+      <groupId>io.dapr.spring</groupId>
+      <artifactId>dapr-spring-boot-observation</artifactId>
       <optional>true</optional>
     </dependency>
     <dependency>

dapr-spring/dapr-spring-boot-4-autoconfigure/src/main/java/io/dapr/spring/boot4/autoconfigure/client/DaprClientSB4AutoConfiguration.java

@@ -22,6 +22,8 @@
 import io.dapr.spring.boot.properties.client.ClientPropertiesDaprConnectionDetails;
 import io.dapr.spring.boot.properties.client.DaprClientProperties;
 import io.dapr.spring.boot.properties.client.DaprConnectionDetails;
+import io.dapr.spring.observation.client.ObservationDaprClient;
+import io.dapr.spring.observation.client.ObservationDaprWorkflowClient;
 import io.dapr.workflows.client.DaprWorkflowClient;
 import io.dapr.workflows.runtime.WorkflowRuntimeBuilder;
 import io.micrometer.observation.ObservationRegistry;