[Studio] Test Prometheus data source connections

[Aias00]

Motivation

This is a small follow-up for #431. The Settings data source test endpoint still returned a fixed success response, so users could not validate whether a configured Prometheus-compatible source was actually reachable.

Changes

• Replace the fixed success response in SettingsService.testDataSource with a real Prometheus-compatible HTTP API probe.
• Query /api/v1/query?query=up for Prometheus, VictoriaMetrics, Thanos, and Mimir style data sources.
• Return explicit failure messages for invalid URLs, upstream Prometheus errors, connection failures, and timeouts.
• Avoid logging the full data source URL during connection tests.
• Add regression tests with a local mock HTTP server.

Non-goals

This PR does not add credential fields or replace dashboard charts. It only makes the existing test endpoint validate the configured Prometheus-compatible URL.

Verification

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -B -ntp -Dtest=SettingsServiceTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -B -ntp test

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[Aias00]

Superseded by #637. Consolidated with related Metrics/Prometheus validation changes to reduce review overhead while preserving the same scoped fixes and tests.