fix(claude): extend thought_signature whitelist to all Gemini Flash models

Gemini 3.5/3.6/3.7 Flash require thought_signature on functionCall
parts, but the whitelist only covered gemini-3-flash / gemini-3.1-flash.
When thinking was disabled as a fallback, multi-turn tool calls from
Claude Code hit HTTP 400:

  Function call is missing a thought_signature in functionCall parts

- Generalize model_keeps_thinking_without_signature() to the whole
  Gemini Flash family (any gemini*flash* id), so the sentinel
  skip_thought_signature_validator is injected even when thinking got
  disabled for those models.
- wrapper.rs: also inject the sentinel when neither a cached signature
  nor a session id is available (previously only covered the empty
  session-cache branch).

Closes #3272 (same error on newer models)