Fix e2e harness compilation against go-github v89 and go-sdk v1.7 (#3187)

* Fix e2e harness compilation against go-github v89 and go-sdk v1.7

The e2e test package no longer compiled under --tags e2e because it
lagged behind two dependency migrations:

- go-github v89: NewClient now returns (*Client, error) and
  WithEnterpriseURLs moved from a *Client method to a
  ClientOptionsFunc. Update getRESTClient and the inline cleanup
  call sites, and drop the trailing nil options arg from
  ListReviewers.

- go-sdk v1.7: ghmcp.NewMCPServer(MCPServerConfig) was replaced by
  ghmcp.NewStdioMCPServer(ctx, github.MCPServerConfig). Update the
  in-process setupMCPClient branch accordingly, supplying the required
  Logger and Version fields.

* fix(e2e): align client host resolution

Reuse the server API host resolver so GHEC REST and upload clients target the correct subdomains. Add token-free coverage for host URL resolution and the in-process stdio server lifecycle.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: ppoffice <8849362+ppoffice@users.noreply.github.com>
Co-authored-by: Sam Morrow <sammorrowdrums@github.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>