refactor(core): consolidate model error mapping

[kitlangton]

Why
Generate maps three ModelResolver failure tags through identical explicit-versus-implicit selection logic. Effect supports catching a non-empty tag list with one narrowed handler.

What Changes
Replaces the three duplicate handlers with one multi-tag catchTag handler. Explicit model requests still fail with ModelSelectionError; implicit resolution still fails with UnavailableError carrying the resolver provider ID.

Scope
The no-model fallback, AI generation error mapping, and separate Integration authorization message remain unchanged. PR #45398 also touches generate.ts for Bedrock region behavior but does not consolidate these resolver handlers.

Verification

cd packages/core
bun typecheck
bun run test test/generate.test.ts test/model-resolver.test.ts
cd ../..
bunx oxlint packages/core/src/generate.ts

Core typecheck passed, all 41 focused tests passed, and oxlint reported no warnings or errors. The push hook completed the 33-package workspace typecheck.