fix(mcp): return structured error responses when tool handlers throw

[Copilot]

When tool handlers threw (e.g., RuntimePreflightError from a failed preflight like missing Shopify CLI), the MCP SDK's internal createToolError was called — which returns { isError: true } with no structuredContent. The stdio smoke test expects structuredContent.error to always be defined when isError is true, causing a TypeError crash.

Summary

• Added toToolError(error) helper in server.ts that converts any thrown exception into a proper structured result:

  { content, structuredContent: { error: { code: "tool_execution_failed", message } }, isError: true }

• Wrapped the four throwable tool callbacks (replicate_site_to_theme, replicate_site_to_hydrogen, get_replication_job, list_replication_jobs) in try/catch using this helper — list_destination_stores is synchronous and not at risk
• list_destination_stores is unaffected (no preflight, no async throws)
• Updated a misleading test description in server.test.ts: "returns a structured runtime preflight error when replication cannot start" → "re-throws runtime preflight errors from the orchestrator without wrapping" — the test exercises the handler layer, not the server wrapper

Verification

• pnpm build
• pnpm test
• pnpm typecheck
• pnpm theme:check

Risk

No API or contract changes. Error structure (structuredContent.error.code) is new surface area on the failure path only — previously clients received undefined structured content on throw, which was a bug.

Original prompt

Run a full check to see if the code is actually runnable. Also explore possible issues that this mcp could have.

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