Avoid duplicate messages in Console logging with Json formatter

[tarekgh]

Fixes #104409

When logging to the console using the JSON formatter, the log message often appears redundantly—once as the top-level Message, again inside the State object, and a third time as the original format string. This commonly occurs in typical usage patterns like logger.LogInformation(...).

This duplication:

• Increases log size
• Causes confusion due to repeated content
• Adds unnecessary overhead by formatting the same message multiple times

Example

logger.LogInformation("This is an information message.");

Current output:

{
  "EventId": 0,
  "LogLevel": "Information",
  "Category": "Program",
  "Message": "This is an information message.",
  "State": {
    "Message": "This is an information message.",
    "{OriginalFormat}": "This is an information message."
  }
}

After the change

{
  "EventId": 0,
  "LogLevel": "Information",
  "Category": "Program",
  "Message": "This is an information message.",
  "State": {
    "{OriginalFormat}": "This is an information message."
  }
}

In addition to removing the redundant Message field from State, I've optimized FormattedLogValues.ToString() to avoid reformatting the message multiple times. This method is frequently called more than once in common scenarios (e.g., LogInformation), resulting in wasted processing. The optimization ensures formatting occurs only once.

Finally, I plan to file a breaking change issue, as this adjustment may affect consumers who rely on the exact shape of the JSON output for parsing or processing.

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.

[tarekgh]

Breaking change issue is filed dotnet/docs#47006 and email sent to the .NET Breaking Change Notification DL.