[moss-orbit-9f339e]

The mod page saved the game mode JSON without a check. A mode with a trailing comma was saved and sent to clients, but Chobby cannot read it, so the mode disappears from the list.
This change parses the JSON with System.Text.Json (strict JSON; the site already gets this package through ZkLobbyServer) on create and edit. It also requires an object with a "name" field, which Chobby needs. The error message gives the line and position. 47 stored modes that Chobby loads are not strict JSON (mostly a missing comma): they keep working, but their next save needs a fix (list in the first comment).
Tested the method under Mono on 54 JSON files against Chobby's real json.lua under Lua 5.1: it rejects every file that Chobby cannot load, except valid JSON 1E5 (Chobby's decoder does not read a capital E). Details are in the first comment.

Fixes #3073

This PR was made with AI help (Claude Code); I checked the change and the test.

🤖 Generated with Claude Code

[moss-orbit-9f339e]

Test details.

Corpus. 54 small files. Each one went through Chobby's LuaMenu/Addons/json.lua (master 982028c3, unchanged, Lua 5.1, with the same loadFile and name steps as GetCustomModes) and through the new GetGameModeJsonError method (copied from the branch, Mono 6.8, System.Text.Json 5.0.0 net461 DLL).

input	Chobby	this PR
trailing comma in object (this issue), in array; double comma	cannot load	rejects
unquoted key; NaN, Infinity, -Infinity; +1; 1e; 0x10; True; undefined	cannot load	rejects
BOM; empty; whitespace only; unterminated object or string; raw newline in string	cannot load	rejects
top-level array, string or null; object without "name"	not listed	rejects
top-level number or true	Lua error in GetCustomModes	rejects
valid modes (CRLF, all value types, \u escapes, duplicate keys, empty key)	loads	accepts
1E5 (valid JSON)	cannot load (no capital E in its number scanner)	accepts
// and /* */ comments; single quotes; missing comma; leading comma; leading zero; .5; 1.; 1-1; number key; text after the object; raw tab or control character; \q \x41 \065 escapes	loads	rejects
valid JSON nested 100 or 1000 deep	loads	rejects (System.Text.Json max depth 64)

Every input that Chobby cannot load is rejected, except 1E5. The last two rows are stricter than Chobby: they are not JSON (or nest far deeper than a mode file).

The issue case gives: Game mode JSON is not valid at line 4, position 1: The JSON object contains a trailing comma at the end which is not supported in this mode. Change the reader options.

Stored modes. I downloaded all 248 modes listed on https://zero-k.info/Mods (8 featured) and ran the same two checks.

• Chobby cannot load 14 of them today. None of them are featured: FW, 11 (its JSON is the number 11, so Chobby raises a Lua error in GetCustomModes while the file is in CustomModes), futurewarsnuclear, zk_constricting_ring, FwMu, VMRTS, X, fortnite, old_no_ai_super, saturdaygaming, TTTEST, AntiChick, chicken_disable_units, testcomma. This PR rejects all 14 at their next save. No mode named siegebots is in the list.
• Chobby loads 47 more that are not valid JSON. 38 have a missing comma, 8 have an extra } at the end, and 1 has a second object after the first. They keep working, but their next save shows the error with its line until the JSON is fixed.

The 47 modes

Test, king_builder, flying_ships, infection, limited_tech, old_random_morphs, 1_unit, old_strider_goo, CAPTURE_SIDEARM, detriment_rush, ulti_spam, capture, zombie_building, chicken_farm_mex, half_price, Flip, i_love_geo, Geo_Flip, wallmap, rampmap, freakmap, sphere, MYAI, diceandsplice, ora, dasor, noregenhermit, randland, BossAI, fallback, ScaleAI, kingAI, chickmex, temple, moreme, terraforma, upgrader, ultimateAI, UltAIGreed, SliceTemple, UltimateAIGEOFlip, UltimateAIsplice, sg5050f, UltimateAIRelick, SKUAI, rampmapMEGA, FORT

• The 8 featured modes all pass.

Build. I could not build the web project here (no .NET Framework web build on this machine). The method was compiled alone with mcs, with the same System.Text.Json 5.0.0 DLL that the site already loads through ZkLobbyServer.