feat(platform/backend): add gemini-3-pro-preview (#11413)

This adds gemini-3-pro-preview from openrouter

https://openrouter.ai/google/gemini-3-pro-preview

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  <!-- Put your test plan here: -->
  - [x]  Test the gemini 3 model in the llm blocks and it works

autogpt_platform/backend/backend/blocks/llm.py

@@ -129,6 +129,7 @@ class LlmModel(str, Enum, metaclass=LlmModelMeta):
     OPENAI_GPT_OSS_120B = "openai/gpt-oss-120b"
     OPENAI_GPT_OSS_20B = "openai/gpt-oss-20b"
     GEMINI_2_5_PRO = "google/gemini-2.5-pro-preview-03-25"
+    GEMINI_3_PRO_PREVIEW = "google/gemini-3-pro-preview"
     GEMINI_2_5_FLASH = "google/gemini-2.5-flash"
     GEMINI_2_0_FLASH = "google/gemini-2.0-flash-001"
     GEMINI_2_5_FLASH_LITE_PREVIEW = "google/gemini-2.5-flash-lite-preview-06-17"
@@ -241,6 +242,7 @@ MODEL_METADATA = {
     LlmModel.OLLAMA_DOLPHIN: ModelMetadata("ollama", 32768, None),
     # https://openrouter.ai/models
     LlmModel.GEMINI_2_5_PRO: ModelMetadata("open_router", 1050000, 8192),
+    LlmModel.GEMINI_3_PRO_PREVIEW: ModelMetadata("open_router", 1048576, 65535),
     LlmModel.GEMINI_2_5_FLASH: ModelMetadata("open_router", 1048576, 65535),
     LlmModel.GEMINI_2_0_FLASH: ModelMetadata("open_router", 1048576, 8192),
     LlmModel.GEMINI_2_5_FLASH_LITE_PREVIEW: ModelMetadata(

autogpt_platform/backend/backend/data/block_cost_config.py

@@ -92,6 +92,7 @@ MODEL_COST: dict[LlmModel, int] = {
     LlmModel.OPENAI_GPT_OSS_120B: 1,
     LlmModel.OPENAI_GPT_OSS_20B: 1,
     LlmModel.GEMINI_2_5_PRO: 4,
+    LlmModel.GEMINI_3_PRO_PREVIEW: 5,
     LlmModel.MISTRAL_NEMO: 1,
     LlmModel.COHERE_COMMAND_R_08_2024: 1,
     LlmModel.COHERE_COMMAND_R_PLUS_08_2024: 3,