Update src/llm/__init__.py

Update model support:

feat: upgrade to GPT-5 series, increase default max_tokens to 8192

fix: Round 14 — 28 Tier 1 + Tier 2 bugs fixed across 17 source files

fix: pre-release audit round 12 — 33 bugs fixed across all tiers

fix: 6-agent debug audit — 8 bugs fixed across codebase

fix: harden codebase against format injection, empty responses, and unsafe config access

fix: use .get() for all config access + add CLAUDE.md and docs to repo

Initial commit: RAG Research Chatbot Template