fix: O11Y-725 - Return no-op span when not initialized (#283)

## Summary
Instead of throwing an `IllegalStateException` when the observability
plugin is not initialized, `startSpan` now returns an invalid span.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Return an invalid no-op span from `startSpan` when not initialized,
plus minor KDoc and formatting updates.
> 
> - **Core behavior**:
> - `companion object` no-op `startSpan` now returns `Span.getInvalid()`
when Observability isn’t initialized.
> - **Docs**:
> - KDoc updated to reference
`com.launchdarkly.observability.plugin.Observability` and streamlined
wording.
> - **Cleanup**:
>   - Minor formatting/style adjustments in `LDObserve.kt`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
9548adf. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: agrognetti

sdk/@launchdarkly/observability-android/lib/src/main/kotlin/com/launchdarkly/observability/sdk/LDObserve.kt

@@ -8,15 +8,15 @@ import io.opentelemetry.api.logs.Severity
 import io.opentelemetry.api.trace.Span
 
 /**
- * LDObserve is the singleton entry point for recording observability data such as 
- * metrics, logs, errors, and traces. It is recommended to use the [Observability] plugin
- * with the LaunchDarkly Android Client SDK, as that will automatically initialize the
- * [LDObserve] singleton instance.
+ * LDObserve is the singleton entry point for recording observability data such as
+ * metrics, logs, errors, and traces. It is recommended to use the [com.launchdarkly.observability.plugin.Observability] plugin
+ * with the LaunchDarkly Android Client SDK, as that will automatically initialize the [LDObserve] singleton instance.
  *
  * @constructor Creates an LDObserve instance with the provided [Observe].
  * @param client The [Observe] to which observability data will be forwarded.
  */
 class LDObserve(private val client: Observe) : Observe {
+
     override fun recordMetric(metric: Metric) {
         client.recordMetric(metric)
     }
@@ -53,7 +53,7 @@ class LDObserve(private val client: Observe) : Observe {
         return client.flush()
     }
 
-    companion object : Observe{
+    companion object : Observe {
         // initially a no-op delegate
         // volatile annotation guarantees multiple threads see the same value after init and none continue using the no-op implementation
         @Volatile
@@ -66,8 +66,7 @@ class LDObserve(private val client: Observe) : Observe {
             override fun recordError(error: Error, attributes: Attributes) {}
             override fun recordLog(message: String, severity: Severity, attributes: Attributes) {}
             override fun startSpan(name: String, attributes: Attributes): Span {
-                // TODO: figure out if a no-op span implementation exists in the otel library
-                throw IllegalStateException("Observability plugin was not initialized before being used.")
+                return Span.getInvalid() // Observability plugin was not initialized before being used.
             }
             override fun flush(): Boolean {
                 return false // No-op, return false to indicate flush was not successful