tsup -> tsdown

[david-crespo]

Let us drop esbuild because as of Vite 8 everything is Rolldown. Had the robot make sure the result was more or less the same.

This will be a patch release for the generator since I don't think it should affect anything for a consumer. For @oxide/api, I probably won't bother publishing it until the next actual release. Though maybe I should do a test release.

🤖 Summary and testing notes

Replaces tsup with tsdown in both @oxide/api and @oxide/openapi-gen-ts. esbuild is now gone from both node_modules and lockfiles — and since vite 8 (via vitest) no longer hard-depends on esbuild either, removing tsup eliminated the last source in the generator package too, not just the API client.

Two config notes:

• fixedExtension: false in both, because tsdown defaults it to true on the node platform, which would emit Api.mjs/index.mjs and break the existing exports/main/bin paths that point at .js.
• tsdown only externalizes declared deps (tsup externalized everything in node_modules). The generator imports @apidevtools/swagger-parser but only declared the thin swagger-parser wrapper, so tsdown bundled it and its whole subtree (710 KB vs 37 KB). Fixed by declaring @apidevtools/swagger-parser directly — it's auto-externalized, ships its own types, and the wrapper was a one-line re-export with no other importers.

Confirmed equivalence by building both packages with old tsup and new tsdown and comparing:

• @oxide/api: identical output filenames (Api.js/Api.cjs/Api.d.ts/Api.d.cts), identical exported names in the .d.ts, identical require() shape ({Api, default} both the class, __esModule: true set by both), and the new .d.ts type-checks clean.
• @oxide/openapi-gen-ts: bundle back to 39 KB, shebang + exec bit preserved, tsc clean, all 138 tests pass. Ran the bundled CLI and the source (tsx) against a real omicron spec — output was byte-identical (diff -rq).