fix(defaults): replace gemini-3-flash-preview and gemini-2.5-flash with gemini-3.5-flash

[hbentel]

Closes #32360.

Problem

Two stale model defaults shipped across the codebase:

• gemini-3-flash-preview — an unversioned preview alias that is undocumented, carries no stability guarantees, and (per issue fix: Fallback to gemini-3-flash-preview fails with 400 (thinking_config) #25123) rejects thinking_config with HTTP 400.
• gemini-2.5-flash — scheduled for shutdown October 16, 2026 per Google's deprecation notice.

The stable replacement for both is gemini-3.5-flash (GA, stable versioned ID).

Changes

Replaces every hardcoded occurrence in:

• Provider plugins: gemini, openrouter, kilocode
• agent/auxiliary_client.py, agent/gemini_native_adapter.py, agent/gemini_cloudcode_adapter.py
• plugins/memory/hindsight/
• trajectory_compressor.py
• hermes_cli/ — config.py, setup.py, models.py, main.py, goals.py
• tools/vision_tools.py, tools/web_tools.py
• datagen-config-examples/trajectory_compression.yaml
• Website docs (English + zh-Hans i18n)
• website/static/api/model-catalog.json (regenerated via scripts/build_model_catalog.py)

Test-only uses of gemini-3-flash-preview that test model-specific routing behavior (e.g. the #25123 regression test that verifies the unversioned preview doesn't receive thinking_config) are intentionally preserved.

Test plan

• scripts/run_tests.sh tests/agent/ tests/tools/ tests/hermes_cli/ tests/cli/ tests/run_agent/ — all pass (pre-existing failures in test_anthropic_adapter, test_gateway_wsl, test_gateway_service, test_service_manager are macOS environment issues unrelated to this change)
• Updated test_provider_parity.py assertions to expect gemini-3.5-flash
• Updated test_auxiliary_client.py assertions for pool and fallback defaults
• Regenerated model-catalog.json; test_model_catalog.py passes

🤖 Generated with Claude Code

[hbentel]

Closing — this PR has been overtaken by main and its broad approach now conflicts with merged maintainer decisions.

Already landed on main via other PRs:

• gemini-3.5-flash added to the catalog and OAuth/API-key pickers (chore(models): gemini-3.5-flash replaces gemini-3-flash-preview in OpenRouter + Nous lists #34581, fix(models): add gemini-3.5-flash to Gemini OAuth + API-key pickers #37046)
• The gemini provider's default_aux_model is already gemini-3.5-flash on main

Conflicts with merged intent: The bulk of this diff replaces gemini-3-flash-preview → gemini-3.5-flash across defaults, pickers, aux models, and docs. But #37606 (merged) deliberately restored gemini-3-flash-preview as a supported model, so removing it wholesale goes against that decision.

Residual value is marginal: what remains that's genuinely still on main and not counter to #37606 is a handful of gemini-2.5-flash fallback defaults (probe default, _create_chat_completion default, cloudcode default, hindsight summarization). gemini-2.5-flash isn't shut down until Oct 16 2026 and these are rarely-hit fallbacks. If that migration is still wanted closer to the deadline, it's better as a fresh, tightly-scoped PR (touching only those defaults, leaving gemini-3-flash-preview intact, and updating the coupled test_gemini_free_tier_setup_block assertion).

Happy to open that focused PR if/when it's useful.