get_issue_details fails with Streamable HTTP error for regional endpoints

## Summary

The `get_issue_details` MCP tool returns a `Streamable HTTP error` when the Sentry issue URL points to a non-default region (e.g. `de.sentry.io`). The error response is an `Invalid content from server` JSON-RPC error, suggesting the proxy or transport layer fails to handle the response correctly for region-specific endpoints.

## Error

```
Error: Streamable HTTP error: Error POSTing to endpoint:
event: message
data: {"jsonrpc":"2.0","id":6,"error":{"code":-32600,"message":"Anthropic Proxy: Invalid content from server","data":null}}
```

## Reproduction

1. Call `get_issue_details` with a Sentry issue URL scoped to an EU/non-default region:
- `issueUrl`: `https://.sentry.io/issues/`
- `regionUrl`: `https://de.sentry.io`
2. Observe the `-32600` JSON-RPC error returned via Streamable HTTP transport.

## Expected behavior

The tool should successfully fetch issue details regardless of which regional Sentry endpoint is targeted. If `regionUrl` is provided, it should be used as the API base and the response should be returned normally.

## Versions affected

- Transport: Streamable HTTP (MCP)
- Region: `de.sentry.io` (EU) confirmed