Fold Go and Zen model catalogs into one parameterized module

[TheGreatAxios]

Summary

• Collapse the Go and Zen catalog bindings into one parameterized module around the shared factory, with identical wire behavior.
• Collapse both catalog test files into one shared behavior harness parameterized by endpoint and discovery functions.
• Extract one bounded-read helper with a single error mapping inside the shared factory.

Verification

• bun run check: exit 0 in an isolated worktree (lint, typecheck, build, full suite under the projects-dir guard).
• Targeted suites after rebase onto latest origin/main: 202 pass, 0 fail across model-catalogs, identity-divergence, provider-setup, and config tests.

Fixes CL-7966

[linear-code]

CL-7966