feat: allow extra client metadata fields in DCR registration

[simonx1]

Summary

Closes #108

Allows passing additional OIDC client metadata fields when constructing an OAuthProvider. These fields are included in the Dynamic Client Registration request and are typically displayed to users on the authorization consent page.

Changes

OAuthProvider

• Added client_metadata: keyword arg to initialize (default: {}). Accepted fields: client_name, client_uri, logo_uri, tos_uri, policy_uri, contacts.
• register_client merges extra metadata into the registration request.

ClientMetadata

• Added optional fields: client_name, client_uri, logo_uri, tos_uri, policy_uri, contacts.
• to_h includes them when set (omits nils via .compact).
• from_h parses them correctly.

Usage

provider = MCPClient::Auth::OAuthProvider.new(
  server_url: "https://mcp.example.com",
  redirect_uri: "https://myapp.com/callback",
  client_metadata: {
    client_name: "My App",
    client_uri: "https://myapp.com",
    logo_uri: "https://myapp.com/logo.png",
    tos_uri: "https://myapp.com/tos",
    policy_uri: "https://myapp.com/privacy"
  }
)

Tests

• client_metadata kwarg is stored and accessible
• Extra fields appear in the DCR registration POST body
• ClientMetadata#to_h includes extra fields when set, omits them when nil
• ClientMetadata.from_h parses extra fields correctly
• All existing specs pass

---

Branch: feature/client-metadata-fields
URL: https://github.com/simonx1/ruby-mcp-client/pull/new/feature/client-metadata-fields

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