Add tests for client name validation with negation scopes

[Copilot]

The issue requested verification that duplicate client name detection works correctly when combining positive scopes with negation scopes (e.g., "python" with "!csharp").

Changes

• Added test verifying conflict detection when scopes overlap:

  @clientName("Foo", "python")    // applies to python only
  @clientName("Foo", "!csharp")   // applies to all except csharp

  Both resolve to "Foo" in python scope → conflict detected ✓

• Added test verifying no conflict when scopes don't overlap:

  @clientName("Foo", "csharp")    // applies to csharp only
  @clientName("Foo", "!csharp")   // excludes csharp

  No overlap → no conflict ✓

Notes

• Existing validation logic in src/validations/types.ts already handles this correctly
• Negation syntax uses ! not ~ (issue description mentioned ~csharp but actual syntax is !csharp)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[TCGC] Client name validation for negation scope case</issue_title>
<issue_description>Current logic could not detect such conflict.</issue_description>

<agent_instructions>Make sure if there are two models with the same client name set, we raise the appropriate error / warning.
For example, if I want to generate something like @@clientName(ModelOne, "Foo", "python") and @@clientName(ModelTwo, "Foo", "~csharp"), I'm not sure we raise the appropriate name-conflict validation. Please check if we do. If we do, add a test and submit the PR. If we do not, update the code so the test / other tests you add work</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [TCGC] Client name validation for negation scope case #2888

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[tadelesh]

The issue aims to solve the problem in this function:

typespec-azure/packages/typespec-client-generator-core/src/validations/types.ts

Line 99 in e80f705

function getDefinedLanguageScopes(program: Program): Set<string | typeof AllScopes> {

.
This function only gathering all positive scopes, but does not take negation scopes into account.
For example, if there is @clientName("XXX", "~csharp"), the validation should apply to AllScopes except csharp.