[RexWHuang]

When literal prompt insertion succeeds but sending Enter fails, tmux_send retries the full operation and inserts the prompt a second time. Track whether text was already sent and return the error without retyping it; keep the existing retry for failures before successful insertion.

Add regression coverage for the Enter failure and for a first-write failure followed by a successful write and failed Enter.

Validation:

• 190 unit tests passed across the Codex helper, loop, manager, log watcher, and real-Codex harness unit suites.
• bash test/smoke.sh passed using Python 3.11.
• The new regression failed before the fix with two text writes instead of one.

Real Codex/tmux integration was not run because tmux is unavailable on the test host. Python 3.10 smoke hits an existing StrEnum import failure, which is outside this patch.