[GitHub Copilot CLI] MCP OAuth Figma token exchange fails with "Failed to parse server response"

## Observed symptom MCP OAuth: Figma token exchange fails with "Failed to parse server response" (distinct from #4870 / #4906) After the `-32601`/`server/discover` fatal-failure bug (#4870) was fixed in 1.0.87-0, the Figma remote MCP server (`https://mcp.figma.com/mcp`) now progresses much further through the OAuth flow — client registration succeeds, the browser consent screen works, and the CLI logs `Completing authentication...` — but the token exchange itself fails with: `OAuth authentication failed for figma: MCPOAuthError: Token exchange failed: Failed to parse server response` ## Literal error text ``` ERROR OAuth authentication failed for figma: MCPOAuthError: Token exchange failed: Failed to parse server response ``` ## Environment GitHub issue github/copilot-cli #4923; state=open ## Trigger / context Public GitHub report in github/copilot-cli (FRESH_14D). ## Source evidence - https://github.com/github/copilot-cli/issues/4923 (2026-09-21T09:11:28Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #4923 ## Freshness - classification: FRESH_14D - retrieved_at: 2026-10-02T09:43:03.388295Z - scout_id: Cf01963d603 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.