Fix CodeQL warnings

[martincostello]

Changes

Fix CodeQL suggestions, which mostly fall into the "minor code analysis warnings" category and a few style ones, plus one bug in the example app.

Merge requirement checklist

• Unit tests added/updated
• CHANGELOG.md updated
• Changes in public API reviewed (if applicable)

[Copilot]

Pull Request Overview

This PR addresses CodeQL warnings by applying minor style and immutability improvements, simplifying conditionals, and fixing a bug in the example app.

• Marked static arrays and settings fields as readonly and reordered override modifiers.
• Replaced explicit == false checks with ! and method-group delegates.
• Updated test code to use using var for activities and added a sample log invocation.
• In the example ASP.NET app, switched from Path.Combine to Path.Join and corrected the created-response ID usage.

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/Grafana.OpenTelemetry.Tests/TracerProviderExtensionsTest.cs	Switched to using var for activity spans
tests/Grafana.OpenTelemetry.Tests/OpenTelemetryLoggerOptionsExtensionsTest.cs	Added a log invocation to the logger test
tests/Grafana.OpenTelemetry.Tests/Grafana.OpenTelemetry.Tests.csproj	Set <LangVersion> to latest
src/Grafana.OpenTelemetry.Base/ResourceDetectors/ResourceDetectorInitializer.cs	Made Initializers readonly and adjusted abstract member order
src/Grafana.OpenTelemetry.Base/ResourceBuilderExtension.cs	Simplified boolean inversion
src/Grafana.OpenTelemetry.Base/Instrumentations/InstrumentationInitializer.cs	Made Initializers readonly
src/Grafana.OpenTelemetry.Base/GrafanaOpenTelemetryResourceDetector.cs	Marked _settings readonly
src/Grafana.OpenTelemetry.Base/ExporterSettings/OtlpExporter.cs	Reordered override modifiers and simplified Enable* checks
src/Grafana.OpenTelemetry.Base/ExporterSettings/GrafanaCloudConfigurationHelper.cs	Made configuration fields readonly
src/Grafana.OpenTelemetry.Base/ExporterSettings/CloudOtlpExporter.cs	Reordered override modifiers and simplified Enable* checks
src/Grafana.OpenTelemetry.Base/ExporterSettings/AgentOtlpExporter.cs	Used method-group delegates and removed redundant casts
examples/net8.0/aspnetcore/TodoAppEndpoints.cs	Switched to Path.Join and fixed the Created response ID

Comments suppressed due to low confidence (1)

tests/Grafana.OpenTelemetry.Tests/OpenTelemetryLoggerOptionsExtensionsTest.cs:26

• Add an assertion to verify that the test logger captured the expected log message to improve test coverage and ensure the logging extension behaves as intended.

            logger.LogInformation("This is a test log message.");