openapi-gen DX: multi-spec config + shared config-core/cli-core refactor (#192 follow-up)

[benjamineckstein]

Follow-up to #192, whose flagship items (--input/--output flags, JS config + defineConfig, --watch) shipped in #236. Remaining DX items deferred from that PR:

• Multi-spec / monorepo: allow a projects: [...] array (or array-of-configs) so one config generates N specs, instead of N configs + N scripts (orval-style).
• Shared config-core / cli-core: config validation + CLI parsing are copy-pasted across openapi-gen, openapi-react-query, and openapi-server. Extract a shared module. (This is the source of the cross-package duplication the fallow audit reports as inherited.)
• .ts config: currently .js/.mjs/.cjs are supported via dynamic import; .ts would need a transpiler (jiti/tsx) - decide whether the dependency is worth it.

Demand-driven; no urgency.

[benjamineckstein]

PR #288 (#288) implements the bulk of this issue and is open with all CI checks green.

What was already in place: The shared config-core/cli-core extraction the issue asked for was already covered by subpath exports of openapi-zod-ts consumed by all downstream packages, so no new package was needed.

What PR #288 adds: Multi-spec projects: [...] array support so one config file drives N spec generations (orval-style). This is delivered via a new normalized loadConfigs() API, a defineProjects() helper, and a shared runProjects iteration kernel (sequential, fail-fast, [i/N] logging). Fully backward compatible, no breaking changes.

Still open / deferred to follow-up:

• The .ts config decision: planned recommendation is to skip jiti/tsx and document the .mjs + defineConfig workaround in a small docs-only PR.
• defineProjects typed helpers for the satellite packages and an integration fixture with committed multi-spec output are noted as future work.

[benjamineckstein]

Shipped in #288 (openapi-zod-ts v1.1.0).