Add Step-03 example agent: add observability using tracing feature and LangFuse (#967)

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## Motivation and Context
Third step of the code agent example.

## Breaking Changes
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#### Type of the changes
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update
- [ ] Tests improvement
- [ ] Refactoring

#### Checklist
- [x] The pull request has a description of the proposed change
- [x] I read the [Contributing
Guidelines](https://github.com/JetBrains/koog/blob/main/CONTRIBUTING.md)
before opening the pull request
- [x] The pull request uses **`develop`** as the base branch
- [ ] Tests for the changes have been added
- [x] All new and existing tests passed

##### Additional steps for pull requests adding a new feature
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Author: denis-domanskii

examples/code-agent/step-03-add-observability/Module.md

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+# Module Code Agent (Step 3)
+
+Code Agent example.

examples/code-agent/step-03-add-observability/README.md

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+# Code Agent - Step 03: Add Observability
+
+> Code from the blog post: [TBA]()
+
+This example extends our Code Agent from Step 2 with OpenTelemetry observability through LangFuse.
+
+## Prerequisites
+
+- Java 17+
+- OpenAI API key
+
+## Setup
+
+```bash
+export OPENAI_API_KEY="your-openai-key"
+export LANGFUSE_HOST="https://cloud.langfuse.com"
+export LANGFUSE_PUBLIC_KEY="your-public-key"
+export LANGFUSE_SECRET_KEY="your-secret-key"
+```
+
+## Run
+
+Navigate to this example:
+```
+cd examples/code-agent/step-03-add-observability
+```
+
+Run the agent on any project:
+```
+./gradlew run --args="/absolute/path/to/project 'Task description'"
+```

examples/code-agent/step-03-add-observability/build.gradle.kts

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+plugins {
+    alias(libs.plugins.kotlin.jvm)
+    alias(libs.plugins.shadow)
+    application
+}
+
+application.mainClass.set("ai.koog.agents.examples.codeagent.step03.MainKt")
+
+dependencies {
+    implementation("ai.koog:koog-agents")
+    implementation(libs.kotlinx.coroutines.core)
+    implementation(libs.logback.classic)
+}
+
+tasks.test {
+    useJUnitPlatform()
+}
+
+tasks.shadowJar {
+    archiveBaseName.set("code-agent")
+    mergeServiceFiles()
+}

examples/code-agent/step-03-add-observability/gradle.properties

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+#Kotlin
+kotlin.code.style=official
+kotlin.daemon.jvmargs=-Xmx4096M
+
+#Gradle
+org.gradle.jvmargs=-Xmx4096M -Dfile.encoding=UTF-8
+org.gradle.parallel=true
+org.gradle.caching=true

examples/code-agent/step-03-add-observability/gradle/libs.versions.toml

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+[versions]
+kotlin = "2.2.20"
+kotlinx-coroutines = "1.10.2"
+kotlinx-serialization = "1.8.1"
+logback = "1.5.13"
+shadow = "9.1.0"
+
+[libraries]
+kotlinx-coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", version.ref = "kotlinx-coroutines" }
+logback-classic = { module = "ch.qos.logback:logback-classic", version.ref = "logback" }
+
+[plugins]
+kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
+kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }
+shadow = { id = "com.gradleup.shadow", version.ref = "shadow" }

examples/code-agent/step-03-add-observability/gradle/wrapper/gradle-wrapper.jar

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.14-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists