Revert adding azure monitor distro in sdkVersion

[rads-1996]

Description

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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Testing Guidelines

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[Copilot]

Pull request overview

This PR reverts use of the Azure Monitor distro environment variable when composing the exporter SDK version, so SDK version reporting falls back to exporter version unless Microsoft OpenTelemetry is explicitly detected.

Changes:

• Removed AZURE_MONITOR_DISTRO_VERSION handling from exporter SDK version generation.
• Updated related SDK version tests.
• Added a changelog entry for the bug fix.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_utils.py	Removes distro env var lookup from _get_sdk_version().
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_constants.py	Removes the unused distro SDK-version environment variable constant.
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_utils.py	Updates SDK version unit tests for the reverted behavior.
sdk/monitor/azure-monitor-opentelemetry-exporter/CHANGELOG.md	Documents the SDK version behavior fix.

Comments suppressed due to low confidence (3)

sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_utils.py:831

• This updated test no longer sets AZURE_MONITOR_DISTRO_VERSION, so it only verifies the default path instead of the regression this PR is intended to cover. Keep the distro environment variable present and assert that _get_sdk_version() still returns the exporter version; otherwise the old dst... behavior could be reintroduced without this test failing.

    @patch.dict(
        "azure.monitor.opentelemetry.exporter._utils.environ",
        clear=True,
    )
    def test_get_sdk_version_distro(self, mock_python_version, mock_prefix):
        result = _utils._get_sdk_version()
        self.assertEqual(result, "uum_py3.11.0:otel1.20.0:ext{}".format(_utils.ext_version))

sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_utils.py:831

• This assertion hard-codes the package's current _version.VERSION without patching azure.monitor.opentelemetry.exporter._utils.ext_version, unlike the default-version test above. The next version bump will make this unrelated test fail unless it is updated, so it should either patch ext_version or compare against the imported value.

        self.assertEqual(result, "uum_py3.11.0:otel1.20.0:ext{}".format(_utils.ext_version))

sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_utils.py:829

• The test name still says distro, but the patched environment is now empty and the assertion covers the default exporter path. This makes the test suite misleading; rename it or restore the distro-env setup so the name matches the behavior under test.

    def test_get_sdk_version_distro(self, mock_python_version, mock_prefix):