Fix fallback logic for extracting connection string from env var

[rads-1996]

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

[Copilot]

Pull request overview

This pull request fixes a bug in the Azure Monitor OpenTelemetry Exporter where the connection string was not properly stored when provided only via the APPLICATIONINSIGHTS_CONNECTION_STRING environment variable. The issue occurred because ConnectionStringParser would read and parse the environment variable but store None in its _connection_string attribute when no connection string was explicitly passed.

Changes:

• Added fallback logic in BaseExporter.__init__ to read from APPLICATIONINSIGHTS_CONNECTION_STRING environment variable when parsed_connection_string._connection_string is None
• Updated CHANGELOG to document the bug fix

Reviewed changes

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

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/export/_base.py	Added fallback logic to read connection string from environment variable when not explicitly provided
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Added entry documenting the bug fix under "Bugs Fixed" section