fix: allow model variants in OAuth Codex filter

[zerone0x]

Summary

• Fixed model variant filtering in Codex OAuth plugin to allow user-defined variants
• Previously only exact model IDs were allowed (e.g., gpt-5.2-codex)
• Now variants like gpt-5.2-codex-low, gpt-5.2-codex-high, gpt-5.2-codex-xhigh are also allowed

Problem

Users who defined custom reasoning effort variants in their opencode.json under the openai provider found that these variants were being filtered out by the Codex plugin's OAuth loader.

Solution

Changed the filter logic from exact Set matching to prefix-based matching.

Fixes #7593