Summary

Closes #185040

Followup to:
#188507
#189519
#190069

This PR contains the last required changed for making Fleet agents write APIs space aware:

• Implement space awareness in the following endpoints:
  • POST /agents/{agentId}/unenroll
  • POST /agents/{agentId}/request_diagnostics
  • POST /agents/bulk_unenroll
  • POST /agents/bulk_request_diagnostics
• Fix a bug in POST /agents/bulk_update_agent_tags where the space id was not passed.
• Simply the setting of the namespaces property when creating agent actions when the space id is known.
• Rename currentNamespace to currentSpaceId where appropriate (see comment below).
• Add API integration tests and consolidate existing ones. ⚠️ At the time of writing, I would like there to be more tests covering bulk query processing in batches, which are currently lacking. I have experienced difficulties getting those tests to pass consistently. Filed followup issue for those.

A note on terminology

As pointed out in #191083 (comment), it seems that the terms "namespace" and "space id" are occasionally used interchangeably in some parts of the codebase to refer to a Kibana space. For instance, documents in Fleet indices (agents, agent policies, agent actions...) possess a namespaces property to track the spaces they belong to. The current space id is also returned using the Saved Object client's getCurrentNamespace function.

However, "namespace" is also a datastream property. In the Agent policy settings UI, the "Spaces" property (which will be linked to the saved object's namespaces property) is above the "Default namespace" property, which relates to the integration's data streams:

This should not be a source of major issues, but is best clarified for future reference. In this PR, I've replaced some occurrences of namespace with spaceId where appropriate to try to maximise the use of the latter.

Testing

• This PR should be put through the Flaky Test Runner prior to merging (I will kick the job).
• Manual testing should also be performed for the new endpoints mentioned above.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• Flaky Test Runner was used on any tests changed