chat: a late task start does not claim failure

[mikemikimike]

Summary

Fixes #1528

Original issue: #1528

Preserve the distinction between an engine refusal and a task start whose reply has not arrived. Ordinary task starts, effort-selected starts, and delegates now carry the existing unanswered-result classification to the chat. The chat warns that the task may already be running instead of claiming it could not start.

Task updates continue through their independent live stream. No automatic retry is introduced, and definite engine refusals retain their existing message. The task manual explains the uncertain result.

Regression Coverage

• An engine accepts one task and reports it on the live task stream while withholding the start receipt beyond the caller's deadline.
• The result remains uncertain, the running task remains visible, and the connection works after the late receipt arrives.
• A definite refusal containing timeout wording remains a definite failure.
• The actual task command and chat update display the uncertain-start message.

Validation

Passed make build, make test-quick, make test TEST_FLAGS=-count=1, and make build-cross on Go 1.26.7. The full test suite passed across all packages, including all eight shards each for internal/session and internal/tui3; cross builds passed for Darwin, Linux, and Windows on amd64 and arm64.