* feat(acp): add reusable MCP connections
- Store named Streamable HTTP connections separately from model providers
- Keep bearer tokens in the credential manager and resolve them per ACP session
- Add localized settings, validation, documentation, and focused coverage
* test(acp): harden MCP connection workflow
- Label credential inputs and confirm destructive connection removal
- Exercise MCP server delivery through an in-memory ACP session
- Extend the browser smoke test to cover accessible input and confirmation flows
* fix(acp): validate MCP connection lifecycle
- Keep non-browser storage initialization in memory and restore the German model copy
- Reject persisted name collisions and invalid draft IDs
- Preserve connections on credential failures and require credentials before enablement
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