Make Model Target dataset downloads more realistic

[adamtheturtle]

Follow-up to #3191.

The Model Target Web API mock currently returns a small valid zip containing request metadata, not a Vuforia-like generated Model Target dataset. Investigate the structure and headers of real standard and advanced dataset downloads, then make the mock return a synthetic artifact that matches the observable shape where practical without committing real credentials or proprietary generated data.

Acceptance criteria:

• Verified fake tests compare response headers and zip structure against real Vuforia downloads.
• The mock returns a deterministic synthetic zip with Vuforia-like filenames and metadata where practical.
• The documented limitation is narrowed or updated to describe any remaining differences.

[adamtheturtle]

Additional clarification after #3191: the current mock-only tests include unknown-dataset download behavior and attempting to download while processing. Unknown-dataset download behavior is externally observable and should become verified fake coverage where practical. The processing-delay case may remain mock-only only if it is specifically testing the mock's deterministic timing control rather than Vuforia's API contract.

As part of this issue, audit download-related mock-only tests and make sure the final suite has no unnecessary mock-only tests: convert or replace externally observable cases with verified fake tests, and document any remaining mock-only cases with the reason they cannot or should not hit real Vuforia.