StylistFunctions.cs: Bare catch in ResolveUserIdFromSessionTokenAsync swallows infrastructure failures

## Problem

`PluckIt.Functions/Functions/StylistFunctions.cs:150-153` uses a bare `catch` with no logging or exception type filter. Infrastructure errors (network, Cosmos, config) are silently returned as `null`, indistinguishable from a legitimately invalid token.

```csharp
catch
{
return null;
}
```

## Impact

Infrastructure outages during session resolution appear as auth failures. Impossible to distinguish from invalid sessions in telemetry.

## Proposed Fix

```csharp
catch (SecurityTokenException) { return null; }
catch (Exception ex)
{
_logger.LogError(ex, "Unexpected error resolving session token.");
throw;
}
```

## Functionality Impact

Previously-silent infrastructure errors surface as 500s. Callers should handle accordingly.