chore(deps): bump ubuntu from 24.04 to 26.04 in /examples/proxy-validation-test (#560)

Bumps ubuntu from 24.04 to 26.04.


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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Example-only Docker and docs changes with no SDK runtime or production
impact.
> 
> **Overview**
> Updates the **proxy-validation-test** example Docker image from
**Ubuntu 24.04** to **26.04**.
> 
> The build installs **`libjitterentropy3-dev`** alongside the existing
toolchain and libraries so the image still builds on the newer base. The
README no longer mentions Ubuntu 24.04 or Boost 1.81+ availability.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
8759831. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Ryan Lamb <4955475+kinyoklion@users.noreply.github.com>

Author: dependabot[bot]

examples/proxy-validation-test/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:24.04
+FROM ubuntu:26.04
 
 # Install dependencies
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
@@ -7,6 +7,7 @@ RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
     ninja-build \
     libboost-all-dev \
     libssl-dev \
+    libjitterentropy3-dev \
     libcurl4-openssl-dev \
     git \
     curl \

examples/proxy-validation-test/README.md

@@ -40,8 +40,6 @@ export LD_MOBILE_KEY="your-mobile-key-here"
 docker compose up --build
 ```
 
-**Note:** The Dockerfile uses Ubuntu 24.04 to ensure Boost 1.81+ is available.
-
 ## Supported Proxy Types
 
 The SDK (via CURL) supports: