Update merge request pipelines table to use real pipeline creation data instead of fake skeleton loader

Overview

Update the merge request pipelines table to use actual pipeline creation data instead of the current "faked" skeleton loader that doesn't persist between page reloads. This is the final step towards making MR pipeline creation and the pipeline table support async pipeline creation.

Current Behavior

When a user clicks "Run pipeline" in a merge request:

1. Frontend shows a temporary skeleton loader row in the pipelines table
2. API call is made to create the pipeline asynchronously
3. Toast message shows "Creating pipeline"
4. Problem: The skeleton loader disappears if the user refreshes the page, even though pipeline creation is still in progress

Desired Behavior

After clicking "Run pipeline":

1. API returns a pipeline creation request ID
2. Pipeline table queries for actual pipeline creation status using this ID
3. Real skeleton loaders persist between page reloads until pipeline creation completes
4. Table updates in real-time as pipeline moves from "creating" → "succeeded/failed"

Technical Details

Current State

• MR pipeline creation uses REST API with async: true but doesn't return request ID
• Backend stores creation requests in Redis via Ci::PipelineCreation::Requests
• GraphQL pipelineCreate mutation exists but doesn't support MR pipelines (missing :merge_request_event source)

Required Changes

Backend:

• Add pipeline_creation_requests field to MergeRequestType using Ci::PipelineCreation::Requests.for_merge_request()
• Either: extend GraphQL pipelineCreate mutation to support MR pipelines, OR return request ID from REST API

Frontend:

• Get pipeline creation request ID from API response
• Query pipelineCreationRequests in get_merge_request_pipelines.query.graphql
• Replace fake isLoading with real status === 'in_progress' in pipelines_table_wrapper.vue

Additional Details

• project.ci_pipeline_creation_request(request_id) returns { status, pipeline_id, error }
• Types::Ci::PipelineCreation::RequestType with status enum in_progress|succeeded|failed