phsl / new-api

A model declared by a task plugin is served only by that plugin's channels.
When the claiming plugin has no enabled channel in the request group, the
distributor answered with the generic "no available channel" text, which hides
the actual cause and led operators to expect channel model_mapping on another
plugin's channel to take over (#7185). That expectation is not supported:
plugin declarations own model names statically, and channel availability must
not silently reassign ownership at request time. The supported fixes are the
existing operator tools, disabling the factory plugin per key or overriding it.

Both no-channel 503 sites in the distributor now route through
noAvailableChannelMessage. When the request is pinned to a task plugin, the
message names the claiming plugin and points to disabling or overriding it;
non-plugin requests keep the generic message. Added in en, zh-CN, zh-TW.