[Feature] Unified proxy response structure to standardize API responses

## Problem Statement

The current sandbox proxy responses lack a consistent structure making integration difficult. The different endpoints return different response structures which complicates client-side handling.

## Proposed Solution

Standardize all API responses to follow consistent structure as described in the new interface response body specification:
- A standardized response wrapper (BaseResponse with status, message, error fields)
- Consistent result objects containing code and failure_reason
- Proper inheritance hierarchy for all response type classes

## Changes Made

- Created specification document at `docs/_specs/response/interface_response_body_spec.md`
- Refactored response classes in `rock/actions/sandbox/response.py` to inherit from `SandboxResponse`
- Added `code` and `failure_reason` fields to all response types
- Updated API endpoint responses to use the `RockResponse[SpecificResponseType]` wrapper
- Added tests in `tests/unit/test_sandbox_response.py` to validate compliance
- Moved interface specification file to proper location for documentation organization

## Benefits

- Consistent API response format makes integration easier
- Clear documentation of response structures
- Better error handling with standardized error fields
- Improved API predictability

## Component Affected

- [x] Sandbox
- [x] Actions
- [x] SDK & API
- [x] Documentation & Examples

Start with docs/_specs/response/interface_response_body_spec.md to understand the response body requirements, then read rock/actions/sandbox/response.py and tests/unit/test_sandbox_response.py. Check the sandbox API endpoint responses and verify that the response classes, fields, wrappers, and inheritance match the specification and that the tests validate compliance.

Written by the indexing model from the issue text.