SDK mocking testkit: operationalize ecosystem-wide slot-in

Parent

Sub-issue of #872 (SDK: ship a fake-transport testkit for consumer-side mocking).

Problem

ADR 0253 vendors the mocking testkit directly into mecatl's own sdk/typescript/ tree for now — no shared package, no dependency on any external mocking-package family, no new sibling package elsewhere. That's the right call to unblock the SDK's own near-term release, but it means no other consumer app can reuse this mocking layer without re-copying it.

Scope

Track the follow-up named in ADR 0253's decision 4: let the mecatl SDK's mocking layer "slot in" to a broader, shared mocker system that other consumer frontend apps might standardize on, rather than mecatl owning a parallel, uncoordinated copy indefinitely. Not yet scoped in depth — this issue is the placeholder for that scoping work.

Priority — not a blocker for the first release

This only matters once a second real Connect-RPC consumer needs the mocking layer. The first mecatl Studio/SDK release ships fine with the vendored, standalone version ADR 0253 describes — this issue tracks later ecosystem work, not a gate on that release.

References

• ADR 0253
• #872 (parent)