fix: also try matching namespaced prefix for modelinfo candidate

[sayan-oai]

What

Try matching \w+-namespaced model after longest prefix as heuristic to match ModelInfo from list of candidates.

This shouldn't regress existing behavior:

• gpt-5.2-codex -> gpt-5.2 if gpt-5.2-codex not present
• gpt-5.3 -> gpt-5 if gpt-5.3 not present
• gpt-9 still doesn't match anything

while being more forgiving for custom prefixes:

• oai/gpt-5.3-codex -> gpt-5.3-codex

Tests

Added unit test.