[Gemini CLI] 400 Bad Request "Requests ending with a model turn are not supported" when reading image files

## Observed symptom bug(core): 400 Bad Request "Requests ending with a model turn are not supported" when reading image files via ReadFile tool When Gemini CLI attempts to inspect image files (e.g. `.png`) using the built-in `ReadFile` tool during an agentic session, the CLI encounters an unhandled API error and enters a broken state with repeated HTTP 400 errors: `✓ ReadFile inspect_history_user.png → Read image file: inspect_history_user.png` ## Literal error text ``` Requests ending with a model turn are not supported ``` ## Environment GitHub issue google-gemini/gemini-cli #29574 ## Trigger / context Public GitHub report in google-gemini/gemini-cli (FRESH_7D). ## Source evidence - https://github.com/google-gemini/gemini-cli/issues/29574 (2026-10-01T03:30:57Z) — evidence_class: PRIMARY_ISSUE_REPORT — #29574 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: Ccac964845f ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.