Fix dotnet-gcdump report command to work with existing dump files

[Copilot]

Issue

The dotnet-gcdump report command was failing when used with an existing dump file:

$ dotnet dotnet-gcdump report fulldump.dmp
Specify only one of -f|--file or -p|--process-id or --dport|--diagnostic-port.

Root Cause

There were two issues:

1. ProcessIdOption was defined as a non-nullable Option<int> but was being used as a nullable (int?) in the code. This caused the condition processId.HasValue to always be true, making the command think both a file and a process ID were specified.
2. The error message referenced a non-existent "-f|--file" option (the file is actually specified as a positional argument, not an option).

Changes

1. Changed ProcessIdOption to be nullable (Option<int?>) to properly support the code logic that checks processId.HasValue.
2. Updated the error message to replace "-f|--file" with "gcdump_filename" to accurately reflect the command syntax.

These changes allow the dotnet-gcdump report fulldump.dmp command to work correctly by properly handling the case when only a dump file is specified.

Fixes #5483.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• cdn.fwupd.org
  • Triggering command: /usr/bin/fwupdmgr refresh (dns block)
• dc.services.visualstudio.com
  • Triggering command: /home/REDACTED/work/diagnostics/diagnostics/.dotnet/dotnet msbuild /m /nologo /clp:Summary /v:minimal /nr:true /warnaserror /p:TreatWarningsAsErrors=true /p:ContinuousIntegrationBuild=false /home/REDACTED/work/diagnostics/diagnostics/artifacts/toolset/restore.proj /t:__WriteToolsetLocation /clp:ErrorsOnly;NoSummary /p:__ToolsetLocationOutputFile=/home/REDACTED/work/diagnostics/diagnostics/artifacts/toolset/10.0.0-beta.25229.109.txt (dns block)
• pkgs.dev.azure.com
  • Triggering command: /home/REDACTED/work/diagnostics/diagnostics/.dotnet/dotnet msbuild /m /nologo /clp:Summary /v:minimal /nr:true /warnaserror /p:TreatWarningsAsErrors=true /p:ContinuousIntegrationBuild=false /home/REDACTED/work/diagnostics/diagnostics/artifacts/toolset/restore.proj /t:__WriteToolsetLocation /clp:ErrorsOnly;NoSummary /p:__ToolsetLocationOutputFile=/home/REDACTED/work/diagnostics/diagnostics/artifacts/toolset/10.0.0-beta.25229.109.txt (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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[hoyosjs]

@dotnet/dotnet-diag - can someone else please review? Looks like initiator can't approve and t requires 2+ reviewers