refactor(unas): replace disable flag with enable, defaulting to off

`disable = false` is a double negative, and a bool named disable cannot
express opt-in anyway: it zero-values to false, so the flag was inert and
opt-in rested entirely on the device list being empty.

`enable` defaults to false and is now the real gate -- Initialize, Metrics
and DebugInput all return early unless it is set. The two existing guards
remain: an empty device list is still a no-op, and no default URL is ever
synthesized.

Configuring devices while enable is false is always a mistake, so that
combination logs one error instead of silently collecting nothing.

Adds binding tests for the flag across toml, json, yaml and UP_UNAS_ENABLE
(the env name derives from the xml tag, not the json one), plus a test that
all three shipped examples default to off. Both were verified by mutation:
breaking a struct tag or flipping an example fails the suite.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/up.conf.example

@@ -275,13 +275,13 @@
 # A UNAS Pro is a standalone UniFi OS console with no Network application, so it is polled
 # by its own input plugin with its own credentials and host -- not as a UniFi controller.
 #
-# This section is entirely optional and does nothing unless you add at least one
-# [[unas.device]] below: with no devices configured the plugin stays silent and inert.
+# This section is entirely optional. The plugin is off unless you set enable = true and
+# add at least one [[unas.device]] below; otherwise it stays silent and inert.
 # Metrics land under the unifi_unas_ prefix in Prometheus, and in the unas_device,
 # unas_pool, unas_disk, and unas_share measurements in InfluxDB.
 #
 #[unas]
-#  disable = false
+#  enable = true
 #
 # Defaults applied to every device that does not set its own value.
 #[unas.defaults]