Use entry Assembly for telemetry version, always learn when learn=true

[alzimmermsft]

What does this PR do?

Two small fixes:

1. Incorrect Assembly was being used to set Version in telemetry. This has been corrected to the entry Assembly, which will now produce the correct Version for telemetry.
2. --mode namespace still had a path where learn=true wouldn't result in learning happening.

GitHub issue number?

Pre-merge Checklist

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[Copilot]

Pull Request Overview

This PR addresses two bugs in the Azure MCP Core:

• Corrects telemetry version reporting by using the entry assembly instead of the calling assembly
• Fixes namespace mode learning to respect the learn=true flag regardless of whether a command is specified

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
core/Azure.Mcp.Core/src/Extensions/OpenTelemetryExtensions.cs	Changed GetServerVersion to SetServerVersion, switched from GetCallingAssembly() to GetEntryAssembly() for accurate version telemetry, and added null safety check
core/Azure.Mcp.Core/src/Areas/Server/Commands/ToolLoading/NamespaceToolLoader.cs	Removed condition that prevented learning when a command was specified, ensuring learn=true always triggers learning mode