test(desktop): add wire.go and sessions.go tests

[HUQIANTAO]

Summary

Add 27 tests for the desktop package's pure-logic functions:

wire.go tests (14 tests)

• toWire: text, reasoning, notice (info/warn), tool dispatch/result, usage (with/without pricing), approval request, ask request, turn_done (with/without error)
• kindNames completeness: all 12 event kinds mapped
• wireEvent JSON round-trip

sessions.go tests (13 tests)

• loadSessionTitles: missing file, corrupt JSON, valid data
• saveSessionTitles: creates nested directory, round-trip
• setSessionTitle: set/clear, whitespace trimming
• deleteSessionFile: with title, without title, missing file
• sessionTitlesPath: correct path
• errActiveSession: has message

Motivation

The desktop package had only dotenv_test.go. The toWire() function (event-to-JSON conversion) and session management (titles, delete) are pure logic testable without Wails runtime. These tests ensure the wire protocol stays consistent and session file operations are correct.