fix(ga): count only consecutive incomplete turns

[Fwind43]

Summary

Count only consecutive incomplete turns toward the three-retry exit limit.

Why

_empty_ct previously accumulated across an entire run. After enough isolated stream failures, a later recoverable stream_read_error could hit the retry limit immediately instead of receiving a fresh retry budget.

The counter now resets to 1 whenever the previous incomplete response was not from the immediately preceding turn.

Testing

• Verified three consecutive incomplete turns still exit on the third attempt
• Verified isolated incomplete turns reset the counter and continue retrying
• Compiled all 72 Python files with py_compile
• git diff --check

Review principles

• Localized to _retry_or_exit in ga.py
• Preserves the existing three-consecutive-failure boundary
• No dependencies or unrelated changes