Fix broken manage_database_backup tool shape in Memory Core OpenAPI

The recent consolidation of backup tools (Issue #8320) introduced a multipart/form-data definition and complex response schemas that broke the Memory Core.

Root Causes:

1. Tool Shape Incompatibility: The multipart/form-data content type and format: binary for the file argument are incompatible with text-based LLM interactions and standard MCP tool definitions.
2. Schema Validation Failure: The use of oneOf in the manage_database_backup response schema caused zod-to-json-schema conversion errors (expected "object") during tool discovery, preventing the server from listing tools.
3. Implementation Mismatch: The DatabaseService.mjs implementation returned inconsistent data structures (e.g., missing message in imports, returning total) that did not align with the defined OpenAPI schema.

Solution:

1. Simplify Input Schema: Change requestBody to application/json and file to a string path.
2. Unified Response Schema: Replace oneOf with a single BackupActionResponse schema containing nullable fields (imported, skipped, total, mode) and a mandatory message.
3. Standardize Service Output: Update DatabaseService.mjs to return a consistent object structure matching the new schema.