Connectors: Make the API more flexible for custom connector types

[gziolo]

Summary

• Validate setting_name, constant_name, and env_var_name in connector registration — reject invalid values with _doing_it_wrong() instead of silently falling back.
• Update PHPDoc to reflect current behavior — widen type from literal 'ai_provider' to non-empty-string, document new authentication fields, use Anthropic and non-AI examples throughout.
• Change 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.
• 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.

Test plan

• Verify existing AI provider connectors retain their connectors_ai_{id}_api_key setting names
• Register a custom non-AI connector type and verify auto-generated setting name uses connectors_{type}_{id}_api_key
• Verify constant_name and env_var_name are stored when provided and omitted when not
• Verify invalid setting_name, constant_name, and env_var_name values trigger _doing_it_wrong()
• Verify API key masking works for non-AI connector types in REST responses
• Verify settings registration skips already-registered settings
• Run full connector test suite: npm run test:php -- --filter=Tests_Connectors

Trac ticket: https://core.trac.wordpress.org/ticket/64957
See also: WordPress/gutenberg#76828

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