MCP `backups_test`: check that the snapshot destination is reachable, as the Backups page's Test button does, for a forge admin #5474

Replaces #5428, the same done started fresh from main. Both tries went red on unrelated tests: the system-stream sampler test (fixed by the issue this waits for) and the asks-mute MCP filter test (#5443).

Done:

• [ ] A tool backups_test takes the fields the page's form sends (destinationInput's shape), admin only, and calls testDestination (src/actions/destination.ts). It answers { reached, says }, with says the action's own sentence; a non-admin is refused as the page refuses. No credential is ever echoed back.
• [ ] test/mcp-backups-test.test.ts covers three cases: a local directory destination is reached; an unreachable one says why; a non-admin is refused. The answer never contains a secret field it was given. Each fails without the tool.
• [ ] One row in docs/MCP.md; a write is marked as one in tools/list's annotations.
• [ ] Before pushing, run test/mcp-backups-test.test.ts, test/mcp.test.ts, test/mcp-docs.test.ts, test/mcp-size.test.ts and bun x tsc --noEmit. A red test elsewhere that main also has is named in the pull, not a reason to stop.

Files: src/http/mcp.ts, docs/MCP.md, test/mcp-backups-test.test.ts. Risk: test/mcp-size.test.ts caps tools/list, so keep the description to one short sentence.

Parked by the crew: the check stayed red through 2 automatic tries (pull #5780 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.