[csharp] Fix compile errors when schema names collide with framework types

[MrD4rkne]

A schema named Parameter, ParameterType, Task, RestRequest, … broke the restsharp ApiClient.cs with CS0104, because it imported the model namespace only to reach AbstractOpenAPISchema. A schema named ValidationResult hid System.ComponentModel.DataAnnotations.ValidationResult inside the model namespace, so every IValidatableObject.Validate implementation stopped compiling.

• restsharp ApiClient.mustache / ApiClient.v790.mustache: drop using {{packageName}}.{{modelPackage}}; and qualify AbstractOpenAPISchema, as the httpclient ApiClient already does.
• validatable.mustache: qualify System.ComponentModel.DataAnnotations.ValidationResult, as ValidateRegex, modelOneOf and modelAnyOf already do. generichost shares this template, so its samples change too; that is most of the sample diff.
• Added 3_0/csharp/model-name-collisions.yaml and two tests in CSharpClientCodegenTest. restsharp and httpclient clients generated from that spec now compile under net8; on master they fail with CS0104 (Parameter, ParameterType, Task) and a Validate return-type mismatch (ValidationResult).

Related: #16123

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cc @mandrean @shibayan @Blackclaws @lucamazzanti

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Summary by cubic

Fixes C# code generation so schemas named like framework types no longer produce code that fails to compile. Previously, a schema named Parameter, ParameterType, Task, or ValidationResult broke the generated restsharp client with CS0104, because ApiClient.cs imported the model namespace and models hid System.ComponentModel.DataAnnotations.ValidationResult.

• Removed the model namespace import from the restsharp ApiClient templates and qualified all AbstractOpenAPISchema references.
• Qualified ValidationResult as System.ComponentModel.DataAnnotations.ValidationResult in the validatable.mustache template shared by restsharp and generichost.
• Added a regression spec and two tests that generate clients from a schema containing these colliding names.

^{Written for commit 9204dfc. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2598 files

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[MrD4rkne]

PR got kinda big (because it touches a lot of *Model.cs and ApiClient.cs. If it is too big, I'm open to suggestions