fix(core): route Copilot fallback models through AI SDK - #44882

Summary

• preserve the aisdk: package discriminator when GitHub Copilot falls back to models.dev catalog entries after model synchronization is unavailable
• verify fallback Copilot models resolve through the bundled Copilot AI SDK instead of attempting to load a nonexistent native package
• cover AI SDK package normalization across synchronized Copilot chat and Anthropic models, models.dev model overrides, legacy V1 config migration, and OpenCode remote provider overrides

Context

Copilot prompts were failing before any provider request with Unsupported package for github-copilot/<model>: @ai-sdk/github-copilot whenever remote Copilot model synchronization failed. The fallback catalog transformer omitted the aisdk: prefix required by model resolution.

Native V2 config handling for bare AI SDK package names is tracked separately in #44881.

Verification

• bun test test/models.test.ts test/model-resolver.test.ts test/config/config.test.ts test/config/provider.test.ts test/plugin/provider-github-copilot.test.ts test/plugin/provider-opencode.test.ts test/github-copilot/models.test.ts from packages/core (127 passing)
• bun typecheck from packages/core
• Prettier formatting check for all changed files