Code-generate the backend mock to eliminate manual sync

[ako]

Problem

Adding a new backend method requires four coordinated edits: interface definition, MPR implementation, mock stub, and compile-time interface check. The mock (mdl/backend/mock/) is hand-written and the most error-prone step — an agent or contributor can miss it, causing compilation failures in unrelated tests.

The mock currently has 17 files for 26 sub-interfaces in FullBackend, indicating it is already drifting from the interface definitions.

Proposed solution

Replace the hand-written mock with one generated by mockgen (or moq) from the backend interfaces. The generated mock is always correct by construction — it can never drift.

Implementation steps

1. Add mockgen or moq as a Go tool dependency (go install in tools.go)
2. Add //go:generate mockgen ... directives to each interface file in mdl/backend/
3. Add make mocks target that runs go generate ./mdl/backend/...
4. Add generated mock files to .gitignore (same rationale as the generated parser: derived from source)
5. Add make mocks step to CI before make test
6. Delete hand-written mock files in mdl/backend/mock/

Expected outcome

Adding a new backend method requires only two edits: the interface and the MPR implementation. The mock is regenerated automatically. Interface drift is impossible.

See proposal: docs/11-proposals/PROPOSAL_agentic_architecture_improvements.md (Change 4)

[github-actions]

Issue Triage

Summary:
The issue proposes to replace the hand‑written backend mock with a generated one (using mockgen or moq) so that adding new backend methods only requires editing the interface and its MPR implementation, eliminating manual sync and drift.

Checklist for feature request:

• ✅ Use case / problem description – provided
• ✅ Proposed syntax or behavior – outlined (add go:generate directives, make mocks target, .gitignore entry, CI step)
• ❌ Mendix version relevance – not specified

Missing information:
Could you let us know whether this change is relevant to a specific Mendix version (e.g., 9.x, 10.x) or if it applies across all versions? Thanks!

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