feat(model-declaration): support per-field help on model provider credentials

[dparkmit24]

Description

Model-provider credential form fields don't carry per-field help, so a plugin manifest's help on such a field is silently dropped during unmarshal and never reaches the provider declaration returned by the management API. Tool credential fields already model this — ProviderConfig (pkg/entities/plugin_entities/config.go) has Help and URL — which is why tool credential help renders in the Dify UI while model-provider credential help does not.

This brings model-provider credentials to parity by adding Help (and URL, matching ProviderConfig) to ModelProviderCredentialFormSchema (pkg/entities/plugin_entities/model_declaration.go). The type Alias unmarshalers handle the new fields automatically. The change is additive and optional — no behavior change for existing plugins.

For example, models/bedrock sets help on bedrock_endpoint_url / bedrock_proxy_url and models/openai on api_protocol; today those never reach the client. (Full trace: manifest → daemon struct → management-API JSON confirms the field is dropped at this struct.)

Type of Change

• Bug fix
• New feature
• Refactor
• Performance improvement
• Other

Essential Checklist

Testing

• I have tested the changes locally and confirmed they work as expected
• I have added unit tests where necessary and they pass successfully

Bug Fix (if applicable)

• I have used GitHub syntax to close the related issue (e.g., Fixes #123 or Closes #123)

Additional Information

This is the first of three coordinated changes needed for a plugin's per-field help to reach the model-provider credential form in the Dify UI: (1) this daemon change so the declaration carries help; (2) the graphon CredentialFormSchema needs the field (it currently drops unknown keys via pydantic extra="ignore"); (3) the Dify frontend needs to map help→tooltip for model-provider credentials (tools/agents already do). Flagging so it's clear this is intentionally scoped to the daemon and is safe/inert on its own.

Note: URL is included for exact parity with ProviderConfig; happy to drop it to Help-only if you'd prefer the minimal change. Similarly, json omitempty was left off help to match ProviderConfig — glad to add it on both if you'd rather.

Drafted with AI assistance (Claude) and reviewed by me before submitting.