velociraptor: get_client_flow_results - optional artifact, no IndexError on multi-source artifacts (fixes #458)

[chiefghost47]

Problem

get_client_flow_results raises IndexError: list index out of range for artifacts with more than
one source, e.g. Generic.Client.Info (#458). Without an artifact= argument, flow_results()
returns no rows for those, and the method returns results[0].

Fix

• api.yaml: an optional artifact parameter on get_client_flow_results (an artifact name, or
  Artifact/Source, e.g. Generic.Client.Info/BasicInformation).
• src/app.py:
  • when artifact is set, it is passed to flow_results() and all rows are returned;
  • when it is empty, the previous return value is kept (the first row), but an empty result now
    returns [] instead of raising IndexError.
• artifact is checked against [A-Za-z0-9_.]+(/[A-Za-z0-9_.]+)? before it is placed in the query,
  so the new input cannot alter the VQL.

There is no behaviour change for existing workflows that get results today.

Testing

Against Velociraptor 0.77.3, calling the app's own methods in an image built with the repo's
Dockerfile:

Case	Before	After
Multi-source artifact (Generic.Client.Info), no artifact	IndexError	returns [], no exception
The same flow with artifact='Generic.Client.Info/BasicInformation'	n/a	returns the rows
artifact value x') FROM scope() --	n/a	refused with ValueError before any query
Single-source artifact (Linux.Sys.Users), no artifact	first row	first row, unchanged

The test image also carried #457 (org_id) so the calls could run in a non-root org. Both PRs change
the get_client_flow_results signature, so whichever is merged second needs a one-line rebase. I'm
happy to do that.

Fixes #458