Add: XPC3004 error diagnostics, to avoid people using RegisterStep<Entity, LocalPluginContext> which fails at runtime.

[mkholt]

This pull request introduces a new analyzer and code fix to prevent a common runtime error when using RegisterStep<TEntity, TService> with LocalPluginContext as the service type. It adds a Roslyn analyzer that detects this misuse at compile time, provides a code fix to automatically rewrite the call to the correct RegisterPluginStep<TEntity> API, and documents the rule and its rationale. Comprehensive tests are included to ensure correct diagnostic and code fix behavior.

Key changes:

Analyzer and Code Fix Implementation

• Added LocalPluginContextAsServiceAnalyzer to report an error (XPC3004) when RegisterStep<TEntity, LocalPluginContext> is used, preventing a runtime exception due to LocalPluginContext not being registered in the DI container. [1] [2]
• Added LocalPluginContextAsServiceCodeFixProvider to provide an automatic fix that rewrites RegisterStep<TEntity, LocalPluginContext> to RegisterPluginStep<TEntity>.

Diagnostics and Rule Documentation

• Registered the new diagnostic rule XPC3004 in AnalyzerReleases.Unshipped.md and documented it in README.md and a new rule file rules/XPC3004.md. [1] [2] [3]
• Added a detailed rule explanation and examples for XPC3004, including both interim and recommended migration paths.

Testing

• Introduced comprehensive unit tests in LocalPluginContextAsServiceAnalyzerTests to verify the analyzer and code fix, including positive and negative cases, code fix correctness, and title.

These changes help catch a subtle but critical error at compile time, improving developer experience and reliability when migrating or writing plugins with dependency injection.

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This fixes #6