Add Validation Rule for path operations to not have a request body

[martincostello]

• Add new OpenApiRecommendedRules class that adds a new GetOperationShouldNotHaveRequestBody rule.
• Avoid using Enter() and Exit() operations in validation rules where possible to avoid stack push/pop calls these perform.
• Use string not String.
• Remove redundant comments.
• Fix typo in comment.

Resolves #2454.

[Copilot]

Pull Request Overview

This PR adds a new validation rule to check that GET operations should not have request bodies, along with several code quality improvements. The changes introduce the OpenApiRecommendedRules class with a specific validation rule for GET operations and refactor existing validation rules to optimize performance.

• Adds GetOperationShouldNotHaveRequestBody validation rule in new OpenApiRecommendedRules class
• Optimizes validation rules by avoiding unnecessary Enter()/Exit() calls when validation conditions aren't met
• Standardizes string usage and fixes typos across validation rule files

Reviewed Changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Microsoft.OpenApi/Validations/Rules/OpenApiRecommendedRules.cs	New class containing the GET operation request body validation rule
test/Microsoft.OpenApi.Tests/Validations/OpenApiRecommendedRulesTests.cs	Comprehensive tests for the new validation rule
test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt	Updates public API surface to include new class
Multiple validation rule files	Performance optimizations and code quality improvements
src/Microsoft.OpenApi/Services/OpenApiVisitorBase.cs	Typo fix in comment

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[baywet]

Thanks for the contribution!

Can you please add a unit test to validate this rule IS NOT present in the default rules (prevent regressions)

[baywet]

Thank you for making the changes!