fix(server): migrate PaperDownloader to PaperMC v3 (fill) API

[ChipWolf]

Problem

PaperMC sunset the v2 API (api.papermc.io/v2); it now returns HTTP 410. PaperDownloader still calls it, so server add paper <version> fails, breaking runServerIntegrationTest (in the build job) and the server-test jobs on every branch and PR:

java.io.IOException: Failed to get https://api.papermc.io/v2/projects/paper/versions/1.21/, response 410:
  {"ok":false,"error":"sunset","message":"This API version has been sunset ..."}
    at ...PaperDownloader.getBuild(PaperDownloader.java:36)

(Purpur, on a different API, still downloads fine in the same test — it's specifically Paper's v2 endpoint that's dead.)

Fix

Switch to PaperMC's current v3 "fill" API (fill.papermc.io/v3). Two shape changes vs v2:

1. GET /v3/projects/paper/versions/{version}/builds returns a top-level array ordered newest-first (v2 returned { "builds": [...] } oldest-first). So the latest build is builds[0], not the last element.
2. Each build carries a ready-made download URL at downloads."server:default".url, so the jar URL no longer needs to be templated.

Verification

• :headlessmc-launcher:compileJava passes.
• Verified against the live API that both code paths resolve correctly: the latest-build path (builds[0]) and an explicit-build lookup both yield a valid server:default URL that serves a real application/java-archive (HTTP 200). v2 confirmed returning 410.
• End-to-end coverage is the existing runServerIntegrationTest (server add paper), which this unblocks.

Context

Surfaced while getting Minecraft 26.2 support green (headlesshq/mc-runtime-test#137, #426). This server-side failure is independent of the 26.2 lwjgl work but was reddening the build check on those PRs.

🤖 Generated with Claude Code

[okafke]

Thanks, will base on this and fix it in a sec. With the animosity many people have towards AI, I would prefer to not have Claude as a contributor in the headlesshq repositories.

Continued in #429