config: migrate velocity.toml to velocity.yaml (Configurate) - #1824

1:1 YAML port of default-velocity.toml with all documentation comments
preserved verbatim. config-version becomes an integer (baseline 1) for
Configurate's versioned transformation system; the `try` list nests under
`servers` to mirror the legacy [servers] table.

Part of the velocity.toml -> velocity.yml Configurate migration.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

…tMapper

Wire up the Configurate ObjectMapper path for the new velocity.yml format:

- Make VelocityConfiguration ObjectMapper-friendly: no-arg constructor,
  non-final nested fields, transient on non-config fields (forwardingSecret,
  motdAsComponent, favicon), and drop the dead gson @expose annotations.
- Annotate Advanced/Query/Metrics @ConfigSerializable and add @setting for the
  keys the lower-case-dashed naming scheme can't derive (kick-existing-players,
  packet-limiter, haproxy-protocol, accepts-transfers, query enabled/port/map).
- Add ConfigurationLoader with a LOWER_CASE_DASHED ObjectMapper factory, a YAML
  loader builder, load/save helpers, and custom TypeSerializers for the dynamic
  sections that don't fit object mapping: Servers (entries + try), ForcedHosts,
  and PacketLimiterConfig (renamed keys).
- Add ConfigurationLoaderTest: loads the bundled default, and round-trips a
  config with non-default values for every renamed/custom-mapped key so a wrong
  mapping can't silently fall back to an identical default.

Part of the velocity.toml -> velocity.yml Configurate migration.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Implement the runtime entry point for the YAML config:

- ConfigurationLoader.loadConfiguration() resolves velocity.yml, migrating a
  legacy velocity.toml or writing the documented default on first start. Absent
  keys fall back to the model's field defaults (matching the old getOrElse
  behaviour), so existing files are never rewritten and their comments survive.
- Migration runs the legacy night-config migrations to normalise the TOML, then
  writes it as YAML stamped config-version=1, preserving a custom
  forwarding-secret-file location, and archives the old file as
  velocity.toml.migrated.
- Forwarding secret resolution mirrors the legacy path (env var, then the
  forwarding-secret-file, creating it if absent) and is injected via a new
  package-private setter; the secret stays out of velocity.yml.

Not yet wired into VelocityServer; that follows in the next change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Route VelocityServer's startup and reload paths through
ConfigurationLoader.loadConfiguration(), so the proxy now reads velocity.yml
(migrating an existing velocity.toml on first start) instead of reading
velocity.toml directly. LegacyConfigurationLoader is retained behind the
migration path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Move the example servers, try order, and forced hosts out of the model's
field defaults and into default-velocity.yml only. Previously, removing a
section caused the loader to substitute the bundled examples, which reference
servers the user may not have and then fail validation. With empty code
defaults, a removed or emptied section now yields an empty collection and the
examples are seeded solely on first-start from the documented default file.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Rename the proxy configuration file and bundled default resource from
velocity.yml/default-velocity.yml to velocity.yaml/default-velocity.yaml.
Third-party plugin descriptor files (plugin.yml, bungee.yml,
paper-plugin.yml) are unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dev/4.0.0 turned ping-passthrough from an enum into a record of five
booleans (version/players/description/favicon/modinfo) with a
night-config migration off the legacy string form.

Mark the record @ConfigSerializable so Configurate's ObjectMapper maps
the section directly (all component names are already single words, so
LOWER_CASE_DASHED needs no @setting overrides), and replace the string
key in default-velocity.yaml with the documented section, mirroring
default-velocity.toml.

The TOML path needs no extra work: PingPassthroughMigration runs as part
of the existing night-config migration chain before the config is dumped
to YAML, so legacy values are expanded on migration.

Tests: the round-trip fixture now carries the section with all five
flags set, and the migration fixture starts from a legacy
ping-passthrough = "DESCRIPTION" and asserts it expands to
description + modinfo.