Add Gemini provider via OpenAI-compatible API

Enables using Google's Gemini models through the OpenAI-compatible API endpoint at generativelanguage.googleapis.com. The provider inherits from OpenAI::ChatProvider, reusing streaming, tool use, and structured output functionality while overriding only Gemini-specific behaviors.

Implementation:
- GeminiProvider inherits OpenAI::ChatProvider, overrides message_merge_delta to fix Gemini's streaming role duplication (Gemini sends role in every chunk, causing "assistantassistant...")
- Gemini::Options handles API key resolution: explicit api_key, access_token alias, then environment variables (GEMINI_API_KEY, GOOGLE_API_KEY in priority order)
- Reuses OpenAI::Chat::RequestType — no protocol translation needed as Gemini implements OpenAI-compatible format
- organization_id and project_id disabled (not used by Gemini API)
- Connection error handling with instrumentation logging

Follows the same pattern established by OllamaProvider which also inherits from OpenAI::ChatProvider for OpenAI-compatible endpoints.

Author: aoki-ryusei

lib/active_agent/providers/gemini/_types.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+require_relative "options"
+require_relative "../open_ai/chat/_types"
+
+module ActiveAgent
+  module Providers
+    module Gemini
+      # Reuse OpenAI Chat request type (same API format)
+      RequestType = OpenAI::Chat::RequestType
+    end
+  end
+end

lib/active_agent/providers/gemini/options.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+require_relative "../open_ai/options"
+
+module ActiveAgent
+  module Providers
+    module Gemini
+      # Configuration options for Gemini provider
+      #
+      # Extends OpenAI::Options with Gemini-specific settings including
+      # the default base URL for Gemini's OpenAI-compatible API endpoint.
+      #
+      # @example Basic configuration
+      #   options = Options.new(api_key: 'your-api-key')
+      #
+      # @example With environment variable
+      #   # Set GEMINI_API_KEY or GOOGLE_API_KEY
+      #   options = Options.new({})
+      #
+      # @see https://ai.google.dev/gemini-api/docs/openai
+      class Options < ActiveAgent::Providers::OpenAI::Options
+        GEMINI_BASE_URL = "https://generativelanguage.googleapis.com/v1beta/openai/"
+
+        attribute :base_url, :string, fallback: GEMINI_BASE_URL
+
+        private
+
+        def resolve_api_key(kwargs)
+          kwargs[:api_key] ||
+            kwargs[:access_token] ||
+            ENV["GEMINI_API_KEY"] ||
+            ENV["GOOGLE_API_KEY"]
+        end
+
+        # Not used as part of Gemini
+        def resolve_organization_id(kwargs) = nil
+        def resolve_project_id(kwargs)      = nil
+      end
+    end
+  end
+end

lib/active_agent/providers/gemini_provider.rb

@@ -0,0 +1,77 @@
+require_relative "_base_provider"
+
+require_gem!(:openai, __FILE__)
+
+require_relative "open_ai_provider"
+require_relative "gemini/_types"
+
+module ActiveAgent
+  module Providers
+    # Provides access to Google's Gemini API via OpenAI-compatible endpoint.
+    #
+    # Extends OpenAI provider to work with Gemini's OpenAI-compatible API,
+    # enabling access to Gemini models through a familiar interface.
+    #
+    # @see OpenAI::ChatProvider
+    # @see https://ai.google.dev/gemini-api/docs/openai
+    class GeminiProvider < OpenAI::ChatProvider
+      # @return [String]
+      def self.service_name
+        "Gemini"
+      end
+
+      # @return [Class]
+      def self.options_klass
+        namespace::Options
+      end
+
+      # @return [ActiveModel::Type::Value]
+      def self.prompt_request_type
+        namespace::RequestType.new
+      end
+
+      protected
+
+      # Executes chat completion request with Gemini-specific error handling.
+      #
+      # @see OpenAI::ChatProvider#api_prompt_execute
+      # @param parameters [Hash]
+      # @return [Object, nil] response object or nil for streaming
+      # @raise [OpenAI::Errors::APIConnectionError] when Gemini API unreachable
+      def api_prompt_execute(parameters)
+        super
+
+      rescue ::OpenAI::Errors::APIConnectionError => exception
+        log_connection_error(exception)
+        raise exception
+      end
+
+      # Merges streaming delta into the message with role cleanup.
+      #
+      # Overrides parent to handle Gemini's role copying behavior which duplicates
+      # the role field in every streaming chunk, requiring manual cleanup to prevent
+      # message corruption.
+      #
+      # @see OpenAI::ChatProvider#message_merge_delta
+      # @param message [Hash]
+      # @param delta [Hash]
+      # @return [Hash]
+      def message_merge_delta(message, delta)
+        message[:role] = delta.delete(:role) if delta[:role]
+
+        hash_merge_delta(message, delta)
+      end
+
+      # Logs connection failures with Gemini API details for debugging.
+      #
+      # @param error [Exception]
+      # @return [void]
+      def log_connection_error(error)
+        instrument("connection_error.provider.active_agent",
+                  uri_base: options.base_url,
+                  exception: error.class,
+                  message: error.message)
+      end
+    end
+  end
+end