test: verify StackdriverLogstashEncoder selects correct MDC trace provider in OTel examples

[Copilot]

PR #450 introduces two MDC JSON providers selected at startup — StackdriverOpenTelemetryTraceMdcJsonProvider (OTel agent) vs StackdriverMicrometerTraceMdcJsonProvider (Spring Boot starter / no agent) — but had no tests asserting which one was actually wired into the encoder.

Changes

• Adds ProviderSelectionTest to all 8 OpenTelemetry example modules (web + gRPC × agent + starter × with/without test artifacts)
• Agent examples: asserts StackdriverOpenTelemetryTraceMdcJsonProvider is present (OTel -javaagent attached via build.gradle)
• Starter examples: asserts StackdriverMicrometerTraceMdcJsonProvider is present (default when isOtelAgent() returns false)

Each test walks the Logback appender tree at runtime to retrieve the live encoder instance, handling both the test-scope CompositeConsoleAppender (via getMachineReadableJsonEncoder()) and the production ConsoleAppender (via getEncoder()):

private static StackdriverLogstashEncoder searchForEncoder(AppenderAttachable<?> attachable) {
    for (var appender : appenders(attachable)) {
        if (appender instanceof CompositeConsoleAppender<?> c) {
            if (c.getMachineReadableJsonEncoder() instanceof StackdriverLogstashEncoder e) return e;
        } else if (appender instanceof ConsoleAppender<?> c) {
            if (c.getEncoder() instanceof StackdriverLogstashEncoder e) return e;
        }
        if (appender instanceof AppenderAttachable<?> nested) {
            var result = searchForEncoder(nested);
            if (result != null) return result;
        }
    }
    return null;
}

Then asserts exactly one provider type is present and the other is absent.