chore(deps-dev): TypeScript 6 + fixes

[mastermanas805]

Bumps the typescript devDependency from 5.9.3 → 6.0.3 (the target of dependabot #16) and fixes the resulting npm run build (tsc) break. Supersedes #16 — that PR fails its build check because the bump alone is not enough on TS 6.

Root cause

TS 6.0 changed ambient type discovery. With this project's NodeNext config, @types/node's global ambient types are no longer auto-included, so every node:-protocol import plus process/Buffer failed at compile time:

src/client.ts(34,30): error TS2591: Cannot find name 'node:fs'. ... add 'node' to the types field in your tsconfig.
src/client.ts(399,7): error TS2591: Cannot find name 'process'. ...
src/client.ts(755,21): error TS2591: Cannot find name 'Buffer'. ...
src/index.ts(45,30): error TS2591: Cannot find name 'node:fs'. ...

(11 errors total across src/client.ts and src/index.ts.)

Fix

• package.json: typescript ^5.9.3 → ^6.0.3 (+ package-lock.json).
• tsconfig.json: add "types": ["node"] to compilerOptions — the exact remedy the TS 6 error message recommends. @types/node@25.9.1 is already a devDependency; this just opts its globals back in. tsconfig.test.json extends this config and inherits the setting, so both the build and the test compile pass.

No strict loosening, no blanket any, no @ts-ignore. The type fix is the canonical one.

Verification

• npm run build (tsc): 0 errors, exit 0.
• npm run build of tsconfig.test.json (the pretest step): 0 errors.
• npm test: 248 pass / 0 fail, coverage 99.81% line / 95.03% branch / 100% funcs (above the 95% floor). The one # SKIP is live smoke (provision-then-teardown), which intentionally self-skips without live network/credentials to api.instanode.dev — expected in CI/sandbox, not a failure.

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