[java/curseforge] Install script fixes and enhancements

[vrrdnt]

Description

Fixes three install-script bugs in the CurseForge generic egg, plus one variable default cleanup.

May resolve:
#121
#127

Commits:

1. [java/curseforge] Fix null/zero serverPackFileId crash on latest install -- Resolves [Install Bug]: Curseforge Generic #121. Adds select(. != null and . != 0) guards in two jq expressions so they stop emitting the literal strings "null" / "0" when CurseForge returns those for serverPackFileId.

2. [java/curseforge] Extract download URL resolution into a helper -- Pure refactor. Consolidates three duplicated /v1/mods/{pid}/files/{fid}/download-url fetches into one cf_resolve_download_url helper. Same endpoint, auth, parsing, and empty-on-failure behavior.

3. [java/curseforge] Surface clear error when API distribution is disabled -- When a project has allowModDistribution: false, /download-url returns 403 with no body. Replace the opaque "No download url found" message at all three call sites with a prominent block that names the cause (author has opted out of API distribution) and tells the user to download the pack manually. For json_download_mods, retain the existing continue behavior but warn loudly that the install will be incomplete. The egg deliberately does not bypass the flag by constructing edge.forgecdn.net URLs from fileId/fileName, as doing so would ignore the author's wishes and risk API key revocation.

4. [java/curseforge] Default modpack File ID to blank -- Variable default change only. The script already treats "" and "latest" identically (both fall back to mainFileId); drop the magic-string default so users get newest-version behavior by leaving the field blank.

5. [java/curseforge] Mirror fixes to pterodactyl egg variant -- The install script between the pelican egg and pterodactyl egg are identical, so the fixes were copied over to the pelican variant's install script.

Tested locally against a real Pelican panel install with several modpacks, including one with allowModDistribution: false (COBBLEVERSE - Pokemon Adventure [Cobblemon], project 1210677) to verify the new error path is reached and the message displays cleanly.

Checklist for all submissions

• Have you followed the guidelines in our Contributing document?

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

• Have you tested and reviewed your changes with confidence that everything works?

• Did you branch your changes and PR from that branch and not from your master branch?

• You verify that the start command applied does not use a shell script

  • If some script is needed then it is part of a current yolk or a PR to add one

• The egg was exported from the panel

[vrrdnt]

Is anyone available to give this a review?