test: direct unit tests for verbose/engaging mode tool event routing

Adds focused unit tests that directly invoke the ToolEventHandler and
IterationCallback closures with a mock bot, proving:

TestVerboseMode_EventSequence:
  Single tool:  💭 sendMessage → 🔧 sendMessage → ✅ sendMessage → delete
  Multi tool:   💭 sendMessage → 🔧×N sendMessage → ✅×N sendMessage → delete×N
  No editMessageText calls in any verbose mode scenario

TestVerboseMode_SameToolMultipleCalls:
  Two read_file calls in one iteration produce 2 tool_call + 2 result
  messages via SendMessage. Detects the known key-collision limitation
  (orphaned first message ID when same tool called twice).

TestEngagingMode_UsesEdits:
  Engaging mode correctly calls editMessageText to update progress.

Author: molty3000