[Shim] Implement StartOperation/StopOperation with W3C trace context support

[Copilot]

Fix Issue # .

Changes

Implements StartOperation extension methods for TelemetryClient to enable manual operation tracking with proper W3C trace context propagation and Activity lifecycle management.

API Surface

Three StartOperation<T> overloads added:

• StartOperation<T>(operationName, operationId, parentOperationId) - Explicit trace/span ID control for distributed tracing scenarios
• StartOperation<T>(operationTelemetry) - Initialize from pre-configured telemetry object
• StartOperation<T>(activity) - Link existing Activity to telemetry (e.g., from message queues, manual instrumentation)

Returns IOperationHolder<T> that auto-tracks telemetry on Dispose().

Core Fixes

Parent ID handling: Only set Context.Operation.ParentId when non-default to prevent invalid "0000000000000000" values on root operations.

Ambient activity suppression: Root operations (no parent span) now suppress Activity.Current during creation to prevent incorrect parent relationships. Suppressed activity restored on dispose if still active.

Operation name consistency: Set on both Activity tags and Context.Operation.Name for reliable retrieval.

Activity integration: Baggage and tags from existing Activity copied to telemetry properties for context preservation.

Supporting Changes

• OperationHolder<T> tracks and restores suppressed ambient activities
• DependencyTelemetry and ExceptionTelemetry initialize Properties dictionary in constructors
• Test suite refactored for clarity and coverage of all overloads
• BasicConsoleApp updated with working examples

Usage Example

// Simple operation tracking
using (var operation = telemetryClient.StartOperation<RequestTelemetry>("ProcessOrder"))
{
    // Inner operations auto-parent
    using (var dep = telemetryClient.StartOperation<DependencyTelemetry>("QueryDatabase"))
    {
        dep.Telemetry.Type = "SQL";
        // ... perform work
    } // Auto-tracked on dispose
}

// Explicit distributed trace context
var traceId = ActivityTraceId.CreateFromString("4bf92f3577b34da6a3ce929d0e0e4736");
var parentSpan = ActivitySpanId.CreateFromString("00f067aa0ba902b7");
using (var op = telemetryClient.StartOperation<RequestTelemetry>(
    "IncomingRequest", 
    traceId.ToHexString(), 
    parentSpan.ToHexString()))
{
    // Maintains distributed trace context from upstream caller
}

// Link existing Activity
var activity = activitySource.StartActivity("CustomOperation", ActivityKind.Server);
using (var op = telemetryClient.StartOperation<RequestTelemetry>(activity))
{
    // Telemetry synchronized with Activity lifecycle
}

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.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.