Change DebugCheck conditional compilation from DEBUG to CHECKED

[Copilot]

Proposed changes

When using null telemetry instances for legacy logging or when logging is unnecessary, DEBUG builds trigger Debug.Fail() assertions in DebugCheck() methods, creating a disruptive debugging experience.

Changed conditional compilation symbol from DEBUG to CHECKED in two DebugCheck() methods:

• LoggerUtils.Null.DebugCheck() in TelemetryUtils.cs
• TelemetryExtensions.DebugCheck() in TelemetryExtensions.cs

Impact: DebugCheck assertions no longer compiled into DEBUG builds. Methods only included when CHECKED symbol is explicitly defined, allowing intentional null telemetry usage without debug interruptions while preserving the ability to track telemetry plumbing issues when needed.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

No additional tests required - existing behavior preserved, only removing debug-time assertions. All 299 existing tests pass.

Original prompt

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This section details on the original issue you should resolve

<issue_title>When providing a null telemetry instance the Opc.Ua.LoggerUtils.Null.DebugCheck triggers consistently during debug</issue_title>
<issue_description>### Type of issue

• Bug
• Enhancement
• Compliance
• Question
• Help wanted

Current Behavior

In cases where logging is not necessary or legacy logging is desired, by providing a null telemetry instance, the Opc.Ua.LoggerUtils.Null.DebugCheck triggers consistently during debug.
This produces a verry annoying debugging experience.

Expected Behavior

A special instance of ITelemetryContext should annul this behavior and offer the legacy logging behavior ?

Steps To Reproduce

No response

Environment

- OS:
- Environment:
- Runtime:
- Nuget Version:
- Component:
- Server:
- Client:

Anything else?

No response</issue_description>

<agent_instructions>There are 2 instances of DebugCheck method that are conditioned using [Conditional("DEBUG")]. Change just the "DEBUG" string to "CHECKED" and nothing more.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes When providing a null telemetry instance the Opc.Ua.LoggerUtils.Null.DebugCheck triggers consistently during debug #3495

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