fix(anvil): use 50-char name limit on 1.17+

[Xuxchloris]

Fixes #3945.

Problem

Minecraft 1.17 raised the anvil custom-name limit from 35 to 50 characters, but openAnvil still rejected names longer than 35 characters on every version, breaking bots that need to rename items with names between 36 and 50 characters.

Change

lib/plugins/anvil.js: compute the limit once per opened anvil from the anvilNameLengthIsFifty minecraft-data feature (bot.supportFeature):

• 1.17+ → 50 characters
• 1.16 and below → 35 characters (unchanged)

Applies to both anvil.combine and anvil.rename.

Tests

test/externalTests/anvil.js: version-gated test that renames an item with a 40-character name (skipped on versions where the limit is still 35). The existing rename tests keep covering the short-name path on every version.

Verification

• standard lint clean on both changed files.
• The new external test runs in the per-version CI matrix on 1.17+; on older versions it early-returns (35-char limit is preserved).

[Xuxchloris]

The failing "MC 1.12.2 1.13.2 1.14.4 1.15.2" job is a pre-existing flake unrelated to this PR:

• The new anvil test early-returns on versions without anvilNameLengthIsFifty (1.16 and below), so it does not execute in this job at all.
• The same job is failing on other open PRs (🎈 26.2 #3926, Add Velocity proxy support for 1.20.2+ configuration phase #3834) that touch completely different code paths.

Could you re-run it? If it still fails I'll dig into the logs.