Negative-path test coverage: validate(), History throw, all tensor stubs, duplicate-add

[petlenz]

Summary

Every intentional error path in the codebase can silently regress without any test failing. None of these are exercised today:

Path	File:Line
validate() rejects an unregistered symbol	include/numsim_codegen/recipe.h:185-204
MooseMaterialTarget throws on InputRole::History	include/numsim_codegen/targets/moose_material.h:40-48
tensor_storage_type rejects rank ∉ {2, 4}	include/numsim_codegen/targets/moose_material.h:259-263
Every NUMSIM_CODEGEN_TENSOR_STUB throw (tensor_mul, basis_change_imp, outer_product, tensor_inv, …)	include/numsim_codegen/code_emit/tensor_code_emit.h:140-150
NUMSIM_CODEGEN_T2S_STUB throw	include/numsim_codegen/code_emit/tensor_to_scalar_code_emit.h:163-167
Compile-check driver tests only the happy path	tests/generated/compile_check_driver.cpp

If the dim check (#7) and duplicate-add check (#8) land, those need coverage too.

Fix

Add tests/NegativePathTest.cpp with a parameterised gtest per row above:

• Assert EXPECT_THROW(model.emit_compute_function(), std::runtime_error).
• Assert the thrown message contains a substring identifying the offending symbol/op.
• For the compile-check driver, add a sibling generator that produces a recipe with an unsupported op (tensor_inv etc.) and assert non-zero exit.

This is the single PR with the highest defensive value — it locks down every intentional guard in one place.

Refs

• REVIEW.md → TEST-A6 (High), CORR-5, CORR-6, CORR-7, CORR-9, CORR-10
• Severity: High

[petlenz]

Partially covered by the NumSim compiler epic #28 — specifically Phase 0.2 (#29) and Phase 1.1 (#30), where the new visitor overrides and stub implementations need their own throw-tests. The remaining negative paths (validate() failure, duplicate-add, etc.) stay scoped to this issue.