Memory leak: MCP OAuth transports not closed on retry or cancellation

[sauerdaniel]

Problem

When MCP OAuth flows are initiated, transports are stored in pendingOAuthTransports Map. These transports are never closed when:

1. OAuth retry - A new transport is created for the same server, orphaning the previous one
2. OAuth cancellation - removeAuth() deletes the map entry but doesn't close the transport
3. OAuth timeout - Incomplete flows leave open transports

Additionally, when the main OpenCode process terminates, spawned MCP server processes may continue running as orphans.

Code Location

packages/opencode/src/mcp/index.ts

Impact

• Open connections accumulate over OAuth retries
• Orphaned MCP server processes after OpenCode exits
• Memory growth in long-running sessions

Relates to #5363

Expected Behavior

• Old transports should be closed before creating new ones
• removeAuth() should close transport before deletion
• Process exit should clean up all MCP clients