Add warning for unused client initialization parameters

[Copilot]

• Explore repository structure and understand the codebase
• Locate relevant files: lib.ts, decorators.ts, package.ts, clients.ts
• Understand how clientInitialization works
• Understand test structure in alternate-type.test.ts
• Add diagnostic warning definition in lib.ts for unused client initialization parameters
• Add validation function to check if clientInitialization parameters are used in operations
• Call validation function in package.ts after client hierarchy is created
• Add comprehensive tests in client-initialization.test.ts
• Add changeset describing the feature
• Fix build failures:
  • Fixed TypeScript compilation errors (return type and invalid kind check)
  • Changed from createDiagnostic to reportDiagnostic to ensure diagnostics are properly reported
  • Fixed validation logic to avoid duplicate warnings for inherited parameters
• All tests passing (82 test files, 1098 tests)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: no warning on unused client initializer param(s)</issue_title>
<issue_description>### Describe the bug

If clientInitialization.parameters contains values that aren't used in any routes (client or its sub-clients), a diagnostic warning should be produced (maybe even an error as this seems like an authoring mistake).

Reproduction

tcgc version 0.54.0

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For bug in the typespec language or core libraries file it in the TypeSpec repo
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.</issue_description>

<agent_instructions>1. Add a linter rule warning definition here https://github.com/Azure/typespec-azure/blob/main/packages/typespec-client-generator-core/src/lib.ts
2. Add verification checks here https://github.com/Azure/typespec-azure/blob/main/packages/typespec-client-generator-core/src/decorators.ts#L1007. If this doesn't work with the test cases later, then add it once the whole client hierarchy is set up
3. Add tests like the one here to check whether a warning is raised https://github.com/Azure/typespec-azure/blob/main/packages/typespec-client-generator-core/test/decorators/alternate-type.test.ts#L199
4. Make sure this works with the @client, @operationGroup, and @clientLocation decorator. For example, make sure the code works if we use @operationGroup decorators to make something a sub client and it has those parameters, that we don't raise a warning.</agent_instructions>

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

• Fixes [Bug]: no warning on unused client initializer param(s) #2549

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[iscai-msft]

@copilot fix build failures

[Copilot]

@copilot fix build failures

Fixed in commit 3a28190. The build failures were caused by:

1. TypeScript compilation error: validation function was returning readonly Diagnostic[] instead of being integrated with the existing diagnostic reporting mechanism
2. Invalid method kind check: attempted to check for "clientaccessor" which doesn't exist in the SdkMethod type union
3. Diagnostics not being reported to the program, so tests couldn't see them

Changes made:

• Switched from createDiagnostic to reportDiagnostic to properly report diagnostics to the program
• Removed invalid clientaccessor kind check
• Fixed validation logic to only validate at top-level clients (not recursively) to avoid duplicate warnings for inherited parameters

All 1098 tests now pass including the 8 new tests for unused client initialization parameter warnings.