VSA requests fail without multiple value streams feature flag

What does this MR do?

Ensures that a value stream id is available before requesting related data for the value stream

• Fallback to the default value stream when no value stream is available
• Introduce the fetchValueStreamData action to group the related request
• Fixes missing duration chart request on initial render

Steps to reproduce

1. Ensure the value_stream_analytics_create_multiple_value_streams feature flag is disabled
2. Navigate to /groups/gitlab-org/-/analytics/value_stream_analytics
3. The stage table and duration chart should load with no failing requests

Screenshots

Feature flag enabled	Feature flag disabled

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #233439 (closed)