Phase out client.tsp separate entrypoint

Phase 1: Import client.tsp from main.tsp
- a. First services that need no changes apart from adding `import "./client.tsp"`
- https://github.com/Azure/azure-rest-api-specs/pull/43749
- https://github.com/Azure/azure-rest-api-specs-pr/pull/28819
- b. Second services that might need simple tweaking (adding !autorest to scope)
- https://github.com/Azure/azure-rest-api-specs/pull/43751
- https://github.com/Azure/azure-rest-api-specs-pr/pull/28820
- https://github.com/Azure/azure-rest-api-specs/pull/44009
- c. more complex services
- https://github.com/Azure/azure-rest-api-specs/pull/44027
```
specification/applink/AppLink.Management
specification/edge/Microsoft.Edge.DisconnectedOperations.Management
specification/frontdoor/resource-manager/Microsoft.Network/FrontDoor
specification/migrate/resource-manager/Microsoft.Migrate/Waves
specification/monitoringservice/resource-manager/Microsoft.Monitor/Agents
specification/napster/Napster.CompanionAPI.Management
specification/programenrollment/resource-manager/Microsoft.ProgramEnrollment/ProgramEnrollment
specification/purviewdatagovernance/data-plane/DataAccess
specification/relationships/Relationships.Management
specification/resources/resource-manager/Microsoft.Authorization/policy
specification/resources/resource-manager/Microsoft.Resources/deploymentStacks
specification/search/resource-manager/Microsoft.Search/Search
specification/storagecache/resource-manager/Microsoft.StorageCache/StorageCache
```

Phase 2: Enforce client.tsp import
- Add tsv validation to make sure client.tsp is imported https://github.com/Azure/azure-rest-api-specs/pull/43762/

Phase 3: Update tooling
- Linting rule to prevent bad pattern in client.tsp https://github.com/Azure/typespec-azure/issues/4652
- Update tsp client and logic in emitters
- https://github.com/Azure/azure-sdk-tools/issues/15955
- cleanup/fix test in typespec-azure reproing the structure
- Documentation
- https://github.com/Azure/typespec-azure/pull/4677
- Should we relax restriction about tcgc needing to be imported in client.tsp only??

Phase 4: Cleanup
- Remove import main.tsp from client.tsp

Start by reviewing the listed specification directories and the linked Phase 1 pull requests to understand the client.tsp migration pattern. Then read the Phase 2 validation work and the Phase 3 tooling, emitter, test, and documentation links. Done means the phased migration, enforced import, tooling updates, documentation, and final cleanup are complete.

Written by the indexing model from the issue text.