Connectors: Fix and generalize the API for custom connector types.
Validate setting_name, constant_name, and env_var_name in connector
registration — reject invalid values with _doing_it_wrong() instead of
silently falling back.
Change the auto-generated setting_name pattern from
connectors_ai_{$id}_api_key to connectors_{$type}_{$id}_api_key so it
works for any connector type. Built-in AI providers infer their names using
the existing connectors_ai_{$id}_api_key convention, preserving backward
compatibility.
Add constant_name and env_var_name as optional authentication fields,
allowing connectors to declare explicit PHP constant and environment
variable names for API key lookup. AI providers auto-generate these using
the {CONSTANT_CASE_ID}_API_KEY convention.
Refactor _wp_connectors_get_api_key_source() to accept explicit
env_var_name and constant_name parameters instead of deriving them from
the provider ID. Environment variable and constant checks are skipped when
not provided.
Generalize REST dispatch, settings registration, and script module data to
work with all connector types, not just ai_provider. Settings
registration skips already-registered settings. Non-AI connectors determine
isConnected based on key source.
Replace isInstalled with pluginFile in script module data output to fix
plugin entity ID resolution on the frontend.
Update PHPDoc to reflect current behavior — widen type from literal
'ai_provider' to non-empty-string, document new authentication fields,
and use Anthropic examples throughout.
Props gziolo, jorgefilipecosta.
Fixes #64957.
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