fix(code): Preserve session state on idle-kill and guard against killing active sessions

[charlesvien]

Idle-killed sessions destroy all chat history, leaving users with a blank screen and no context on retry. Sessions with in-flight MCP tool calls can also be killed since killIdleSession only checks promptPending.

1. Add inFlightMcpToolCalls.size > 0 check to killIdleSession so sessions with active MCP tools are never idle-killed
2. Add same check to hasActiveSessions for consistency
3. Replace teardownSession with handleIdleKill that preserves events, logUrl and config via updateSession instead of removing the session
4. Add tests for idle-kill guard when MCP tool calls are in-flight

Test plan

• All 558 existing tests pass
• New tests for inFlightMcpToolCalls idle-kill guard
• Manual: trigger idle-kill → verify error overlay with history preserved → Retry reconnects with context