Add a potential sub-rule to 'SdkTspConfigValidation' rule for csharp emitter in TSV

The SDK validation for .NET currently relies on the presence of the '@azure-typespec/http-client-csharp-mgmt' emitter option in a spec's 'tspconfig.yaml'. However, if this emitter option is omitted from 'tspconfig.yaml', SDK validation is skipped. This creates a coverage gap and could result in PRs bypassing necessary validation steps.

Proposal: Consider adding a sub-rule to 'SdkTspConfigValidation' in TSV for csharp emitter, which would flag spec PRs that omit the emitter option. This would help ensure all relevant specs are validated, but it might also enforce the requirement for all the specs using legacy emitter.

Is there a better approach to address this scenario and alert maintainers when emitter settings are missing?

@MaryGao CC: @maririos @lirenhe @live1206

Start by locating the SdkTspConfigValidation rule in TSV and reviewing how it detects the @azure-typespec/http-client-csharp-mgmt emitter in tspconfig.yaml. Compare behavior for specs using the csharp emitter with and without that option, then determine how a missing setting should be reported without unintentionally enforcing it on legacy emitter specs.

Written by the indexing model from the issue text.