Add User-Agent property to Serilog logging (#34)

## Overview
This PR enhances Serilog logging by including the User-Agent header from
HTTP requests as a property in log events, improving traceability and
diagnostics.

## Changes
Updated `HttpContextLogEventEnricher` to extract the User-Agent header
from incoming HTTP requests and add it as a property named "User-Agent"
to all relevant Serilog log events.

## Implementation Details
The implementation follows the existing pattern used for other HTTP
context enrichment in the codebase:
- Checks if the User-Agent header exists in the HTTP request using
`httpContext.Request?.Headers?.TryGetValue()`
- Only adds the property when the header is present (omitted when
absent)
- Uses `AddPropertyIfAbsent()` to avoid conflicts if the property is
already set
- Properly handles null checks with null-conditional operators

```csharp
if (httpContext.Request?.Headers?.TryGetValue("User-Agent", out var userAgent) == true)
{
    logEvent.AddPropertyIfAbsent(propertyFactory.CreateProperty("User-Agent", userAgent.ToString()));
}
```

## Benefits
- **Improved diagnostics**: Easier identification of request sources for
troubleshooting
- **Traffic analysis**: Better visibility into client types and patterns
- **Security auditing**: Enhanced log information for security reviews
- **Consistency**: Logs now include standard HTTP metadata alongside
existing properties like UserName

## Testing
- ✅ Solution builds successfully with no errors or warnings
- ✅ Code formatting follows project standards defined in `.editorconfig`
- ✅ Implementation matches the pattern used in other services (e.g.,
`TimeZoneService`)

Fixes issue regarding User-Agent property in Serilog logging.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Add "User-Agent" property to Serilog
logging</issue_title>
> <issue_description>Enhance Serilog logging by including a property
named "User-Agent" that contains the User Agent string from the HTTP
request, if available. This will improve traceability and diagnostics by
identifying the source of requests.
> 
> **Proposed Solution:**
> - Update the logging logic to extract the User Agent header from the
HTTP request (if it exists).
> - Add this value as a property named "User-Agent" in all relevant
Serilog log events.
> 
> **Benefits:**
> - Easier analysis of traffic patterns and troubleshooting.
> - More informative logs for auditing and security review.
> 
> **Acceptance Criteria:**
> - The "User-Agent" property is present in log entries when an HTTP
request includes a User Agent header.
> - If the header is absent, the property should be omitted or set to
null/empty.
> </issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>
Fixes #33

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Author: marcominerva

src/PdfSmith/Logging/HttpContextLogEventEnricher.cs

@@ -15,5 +15,10 @@ public void Enrich(LogEvent logEvent, ILogEventPropertyFactory propertyFactory)
         }
 
         logEvent.AddPropertyIfAbsent(propertyFactory.CreateProperty("UserName", httpContext.User?.Identity?.Name));
+
+        if (httpContext.Request?.Headers?.TryGetValue("User-Agent", out var userAgent) == true)
+        {
+            logEvent.AddPropertyIfAbsent(propertyFactory.CreateProperty("User-Agent", userAgent.ToString()));
+        }
     }
 }