fix(core): validate public session model switches

[kitlangton]

Summary

• validate public Session model switches against the Session's exact Location catalog after plugin boot
• reject unavailable models and explicit unknown variants with typed public errors before durable mutation
• preserve Session.NotFoundError and keep SessionV2.switchModel as the durable event operation

Tests

• available model and variant succeeds
• missing and disabled models reject without mutation
• exact Location catalog is used
• unknown variant rejects without mutation
• missing Session preserves Session.NotFoundError

Verification

• bun run test test/public-opencode.test.ts (isolated database)
• bun typecheck (packages/core)
• pre-push full workspace typecheck (22 packages)
• bunx prettier --write on changed files
• git diff --check