Connection String env var - base, web, nlog

[harsimar]

This PR adds code to fall back to the env var for connection string if it isn't set via code for the BASE, WEB, NLogTarget packages.

Tested by setting a system env var then running example apps in repo with code config for connection string removed.

[Copilot]

Pull request overview

Adds connection-string resolution via the standard APPLICATIONINSIGHTS_CONNECTION_STRING environment variable when a connection string isn’t provided programmatically, aligning BASE and NLog target behavior with common Azure Monitor configuration patterns.

Changes:

• NLog target (ApplicationInsightsTarget) now falls back to APPLICATIONINSIGHTS_CONNECTION_STRING if the configured layout renders empty/whitespace.
• BASE TelemetryConfiguration.Build() now reads APPLICATIONINSIGHTS_CONNECTION_STRING when ConnectionString was not set in code, and avoids setting exporter options when the value is empty.
• BASE InitializeFeatureReporter() now short-circuits when connectionString is empty (but this introduces a correctness risk; see comments).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
LOGGING/src/NLogTarget/ApplicationInsightsTarget.cs	Adds env-var fallback for the NLog target connection string during initialization.
BASE/src/Microsoft.ApplicationInsights/Extensibility/TelemetryConfiguration.cs	Adds env-var fallback during OTel SDK build and adjusts exporter option assignment / feature reporter initialization.

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[mattsains-msft]

Other than the first LLM-generated comment, I think this PR is good to go. Will approve after that's resolved