fix: route prefix and non-word-character prep topics to the AI mentor

[ionfwsrijan]

Problem

backend/utils/domainClassifier.js builds the keyword matcher as new RegExp(\b(<keywords>)\b, 'i'). Two classes of keywords could never match, so legitimate AI-mentor questions were refused as "off-topic":

1. Partial-word keywords — scal (scalability), load balanc (load balancing), negotiat (negotiation), probab (probability), math (mathematics), quant (quantitative). The trailing \b requires a non-word char immediately after the prefix, which never happens inside a longer real word.
2. Keywords with non-word characters — c++/c#. The trailing \b after +/# rarely matches ("What is C++?").

isPrepPilotDomain gates the model call in aiRoutes.js, so these prompts got the canned refusal instead of an answer.

Fix

Anchored keywords as prefixes: keep the leading word boundary, drop the trailing \b. \b(probab|scal|...|c\+\+|c\#) now matches "probability", "scaling", "C++", "C#", etc. A few extra matches (e.g. "google" → go) are acceptable for an AI-mentor router.

Files changed

• backend/utils/domainClassifier.js — dropped trailing \b from keywordRegex.
• backend/tests/domainClassifier.prefixKeywords.unit.test.js — tests routing probability/load balancing/scaling/C++/C#/negotiation/mathematics to the mentor and still rejecting clearly off-topic prompts.

Testing

npx vitest run tests/domainClassifier.prefixKeywords.unit.test.js — 13/13 passing.

Closes #1440

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📥 Commits

Reviewing files that changed from the base of the PR and between 8acb5b8 and 1d89905.

📒 Files selected for processing (2)

• backend/tests/domainClassifier.prefixKeywords.unit.test.js
• backend/utils/domainClassifier.js

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