refactor(translator): streamline Codex response handling and remove redundant code
- Updated `ConvertCodexResponseToOpenAIResponses` logic for clarity and consistency.
- Simplified `ConvertCodexResponseToOpenAIResponsesNonStream` by removing unnecessary buffer setup and scanner logic.
- Switched to using `sjson.SetRaw` for improved processing of raw input strings.
- Added new "Gemini 2.5 Flash Image Preview" model definition, with enhanced image generation capabilities.
- Increased scanner buffer size to 20,971,520 bytes across executors and translators to handle larger payloads.
- Added `examples/custom-provider/main.go` showcasing custom executor and translator integration using the SDK.
- Removed redundant debug logs from translator modules to enhance code cleanliness.
- Updated SDK documentation with new usage and advanced examples.
- Expanded the management API with new endpoints, including request logging and GPT-5 Codex features.
- Eliminated unnecessary calls to `misc.CodexInstructions` and corresponding checks in response processing.
- Streamlined instruction handling by directly updating "instructions" field from the original request payload.
- Added `CodexInstructions(modelName string)` function to dynamically select instructions based on the model (e.g., GPT-5 Codex).
- Introduced `gpt_5_instructions.txt` and `gpt_5_codex_instructions.txt` for respective model configurations.
- Updated translators to pass `modelName` and use the new instruction logic.
- Updated all `github.com/luispater/CLIProxyAPI/internal/...` imports to point to `github.com/luispater/CLIProxyAPI/v5/internal/...`.
- Adjusted `go.mod` to specify `module github.com/luispater/CLIProxyAPI/v5`.
