fix(concepts): reject LLM refusal definitions + blank concept name (#2440)

[jung-thomas]

Problem (#2440)

The concept-definition batch backfill approved ~9,290 degenerate definitions to prod — fluent LLM refusals ("the concept name was not provided…", "the concept provided is undefined", "sources do not contain sufficient information about X") that passed the length + terminology guardrails and were bulk-approved. On-page they render as refusal text instead of definitions; one even rendered the literal string "undefined".

Post-incident investigation also found the original #2426 500-run was NOT clean — it contained 26 "valid name, thin grounding" refusals too.

Root cause

Two gaps, both needed:

1. No semantic guardrail. applyPostValidation only checked length + stale terminology. A fluent refusal is non-null, in-range, clean-termed → counted as generated and bulk-approved. The "9,290 generated / 0 errors" metrics were false positives.
2. Blank concept name reached the model. Some generation path fed CONCEPT: undefined into the prompt, so the model correctly refused. (The shipped CQL job passes a correct lowercase concept.name — verified via kg-concept-loader.js's dual-branch pattern — so this was not the shipped job, but the guardrail catches the output regardless of source.)

Fix

• applyPostValidation rejects refusal signatures → reason:'refusal' (never written, never approved). Patterns cover every phrasing observed live on prod.
• generateConceptDefinition refuses a blank/undefined concept.name → reason:'no-name', failing loud instead of emitting CONCEPT: undefined.
• Verified genuine definitions (Data Product Catalog, webhook, sap.m.StandardListItem, UI5 Value State) still pass — no false positives.

Prod data remediation (done separately, out-of-band)

• Rolled back all 9,290 batch-2440 rows (marker-based) → NULL. 0 remain.
• Rolled back the 26 degenerate refusals in the Topics and Concepts #2426 set (by exact ID) → NULL. 474 good Topics and Concepts #2426 rows intact, 0 refusals left.
• Pending: PROD content rebuild so pages stop rendering the garbage and fall back to tiles.

Tests

test/unit/concept-definition-extract.test.js — 12 pass. New cases cover refusal signatures (incl. real prod phrasings), the no-name guard, and no-false-positive checks.