fix NPE with feature reporter not being available

[Matthew Sainsbury (Microsoft) (mattsains-msft)]

fixes #3197

Changes

FeatureReporter was being called without checking if it is null (for example, when a connection string is not set)

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

[Copilot]

Pull request overview

This PR addresses crashes when TelemetryConfiguration.FeatureReporter is not initialized (notably when no connection string is configured), by making feature-reporting calls null-safe across TelemetryClient and TelemetryClientExtensions.

Changes:

• Guard FeatureReporter.MarkFeatureInUse(...) calls with the null-conditional operator (?.) to prevent NullReferenceException.
• Apply the same null-safety pattern broadly across several TelemetryClient.Track* APIs and StartOperation overloads.
• Add a changelog entry documenting the fix (with a small wording nit noted in review comments).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
CHANGELOG.md	Documents the fix for #3197 (wording can be tightened).
BASE/src/Microsoft.ApplicationInsights/TelemetryClientExtensions.cs	Prevents StartOperation from throwing when FeatureReporter is null.
BASE/src/Microsoft.ApplicationInsights/TelemetryClient.cs	Prevents multiple Track* APIs from throwing when FeatureReporter is null.

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