feat: Enable project-level exceptions in ReferenceCop ruleset

[mfogliatto]

Summary

Adds support for configuring project-level exceptions on rules in the ReferenceCop config file. Projects listed in a rule's <Exceptions> element are exempt from that rule.

Changes

• ReferenceCopConfig.cs: Added ProjectException class and Exceptions list to Rule base class, with IsProjectExempt() method
• AssemblyNameViolationDetector.cs: Accepts optional projectName parameter; checks exceptions before reporting violations
• ProjectTagViolationDetector.cs: Checks exceptions using project file name
• ProjectPathViolationDetector.cs: Checks exceptions using project file name
• ReferenceCopAnalyzer.cs: Passes compilation.AssemblyName to the detector
• ReferenceCopConfig.xsd: Updated schema with Exceptions element for all rule types
• ProjectExceptionTests.cs: Unit tests for the new functionality

Usage

<Rules>
  <AssemblyName>
    <Name>NoSystemXml</Name>
    <Description>System.Xml is forbidden</Description>
    <Severity>Error</Severity>
    <Pattern>System.Xml</Pattern>
    <Exceptions>
      <Project Name="LegacyAdapter" />
      <Project Name="XmlTools" />
    </Exceptions>
  </AssemblyName>
</Rules>

Fixes #29

[mfogliatto]

Pending on CI/CD change first.