fix(opencode): terminate local MCP process trees

[YidiDev]

Summary

• reuse the existing descendant-process cleanup whenever a connected MCP client is released
• apply it to disconnect, replacement, and post-connect initialization failure
• add a real stdio regression covering parent and child termination for replacement and disconnect

Verification

• bun test test/mcp/lifecycle.test.ts --timeout 30000 (22 pass)
• bun run typecheck
• Prettier and git diff --check

Closes #46253
Related to #46035

[github-actions]

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[github-actions]

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[YidiDev]

Resubmitted with the required PR template as #46312.