Refactor MCP transport behind remote data source

[ZeiadT]

Summary

• Move Retrofit ownership back into :core:network.
• Add an MCP remote data source that owns API calls and shared error mapping.
• Update McpRepositoryImpl and tests to depend on the remote data source.

Verification

• :core:data:testDebugUnitTest passes.
• Retrofit-leak scan and git diff --check pass.
• Full app assembly requires the local �pp/google-services.json, which is not present in the isolated worktree.