[Router] Require classifier runtime weights so partial downloads heal - #2820

Closes #2669

Purpose

On the mmBERT-32K backend, which is the default for all three, the category, PII, and jailbreak classifiers load through TraditionalModernBertTokenClassifier::new_with_variant. It hard-reads config.json, tokenizer.json, and model.safetensors from the model root and fails outright if any is missing

Completeness only demanded config.json plus any weight-shaped file, and hasModelWeights matches *.safetensors, *.bin, and *.onnx recursively. So an interrupted download that left the companion mapping and a nested adapter blob behind read as complete, was never re-fetched, and since these classifiers register with bestEffort: false, every restart failed the same way

addClassifierModelRequiredFiles mirrors the existing addEmbeddingModelRequiredFiles from #2172 and records the root weights and tokenizer for the three models. Module affected: Router

On the LoRA caveat raised in the issue: this is gated on use_mmbert_32k, which is exactly the flag selecting the initialiser that has no LoRA fallback. With it off, PII and jailbreak init auto-detects LoRA directories that legitimately carry adapter weights instead of root weights, so those keep the looser heuristic. A test pins that carve-out

Test Plan

go test ./pkg/modeldownload/ -count=1
make test-semantic-router
make go-lint
make check-go-mod-tidy
make agent-ci-lint CHANGED_FILES="src/semantic-router/pkg/modeldownload/config_parser.go,src/semantic-router/pkg/modeldownload/classifier_completeness_test.go"

To confirm the tests are not vacuous, revert each part of the fix and re-run

Test Result

All four new tests fail on the base for the right reason, for example:

--- FAIL: TestBuildModelSpecsRequiresClassifierRuntimeWeights
    models/mmbert32k-pii-detector-merged RequiredFiles = []string{"config.json", "pii_type_mapping.json"}, missing "model.safetensors"
--- FAIL: TestPartialClassifierDirReportedIncomplete
    partial classifier dir reported complete; the runtime hard-loads model.safetensors and would fail at init

Reverting each part of the change in turn fails at least one test every time:

make go-lint reports 0 issues, make check-go-mod-tidy and make agent-ci-lint pass, and every applicable pre-commit hook passes. make test-semantic-router reports one failure, TestHybridCachePendingRequest in pkg/cache, which is a known flake unrelated to this change. It fails on other contributors' PRs that do not touch pkg/cache either (#2530, #2507, #2810), and it passed on my own PR #2819. The test writes to the Milvus-backed hybrid cache, sleeps a fixed 100ms "for indexing", then asserts the entry is findable, so a loaded runner fails it. Locally it cannot run at all without a Milvus instance, which is why make test-semantic-router defaults SKIP_MILVUS_TESTS=true. It is currently failing on several unrelated PRs and on main itself

One thing I did not change: DefaultRequiredFiles still allows the loose nested-weight heuristic for every other model. Tightening that globally would be a much wider change than this issue

The models/ prefix is now a named constant, since the linter flags the third repetition this change would have introduced