refactor(onlineexperimentation): migrate to unified folder structure

[zedy-wj]

This PR migrates the onlineexperimentation specification to the new unified folder structure according to Azure folder structure guidelines.

Changes:

• Consolidated TypeSpec and Swagger files under resource-manager/Microsoft.OnlineExperimentation/OnlineExperimentation
• Consolidated TypeSpec and Swagger files under data-plane/OnlineExperimentation
• Updated tspconfig.yaml with correct paths.
• Fixed input-file paths in readme.md
• Verified TypeSpec compilation works correctly

Benefits:

• Unified structure for better maintainability
• Aligns with Azure folder structure standards
• Maintains backward compatibility
• Supports future TypeSpec migration efforts

[github-actions]

Next Steps to Merge

Next steps that must be taken to merge this PR:

• ❌ This PR targets either the main branch of the public specs repo or the RPSaaSMaster branch of the private specs repo. These branches are not intended for iterative development. Therefore, you must acknowledge you understand that after this PR is merged, the APIs are considered shipped to Azure customers. Any further attempts at in-place modifications to the APIs will be subject to Azure's versioning and breaking change policies. Additionally, for control plane APIs, you must acknowledge that you are following all the best practices documented by ARM at aka.ms/armapibestpractices. If you do intend to release the APIs to your customers by merging this PR, add the PublishToCustomers label to your PR in acknowledgement of the above. Otherwise, retarget this PR onto a feature branch, i.e. with prefix release- (see aka.ms/azsdk/api-versions#release--branches).
• ❌ This PR has at least one change violating Azure versioning policy (label: VersioningReviewRequired).
  To unblock this PR, either a) introduce a new API version with these changes instead of modifying an existing API version, or b) follow the process at aka.ms/brch.
• ❌ The required check named Swagger Avocado has failed. Refer to the check in the PR's 'Checks' tab for details on how to fix it and consult the aka.ms/ci-fix guide

Comment generated by summarize-checks workflow run.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

Language	API Review for Package
Swagger	Microsoft.OnlineExperimentation-OnlineExperimentation
Swagger	OnlineExperimentation-OnlineExperimentation
TypeSpec	Microsoft.OnlineExperimentation
Go	sdk/analytics/azonlineexperimentation
Go	sdk/resourcemanager/onlineexperimentation/armonlineexperimentation
Java	com.azure.resourcemanager:azure-resourcemanager-onlineexperimentation
JavaScript	@azure-rest/onlineexperimentation
JavaScript	@azure/arm-onlineexperimentation