feat: add openai_config.reasoning_model override and bump fantasy - #29230

Why

The coder/fantasy fork classifies OpenAI reasoning models by name. The pinned version did not know gpt-6-astra, so the Responses client omitted the reasoning parameter (the reasoning effort picker did nothing) and forwarded temperature (quickgen title calls got HTTP 400 Unsupported parameter: 'temperature' and retried). Every new model generation would need a fork release to fix this. Admins need a switch in the model config instead.

What

• Bump charm.land/fantasy to coder/fantasy f5b3f253c112 (feat(providers/openai): allow reasoning-model overrides and recognize gpt-6+ fantasy#57): gpt-6 and later generations are recognized as reasoning models, and the provider gains WithReasoningModelFunc to override the classification.
• New tri-state openai_config.reasoning_model on ChatModelCallConfig: unset keeps fantasy's name heuristics, true forces reasoning-model handling (effort and summary sent, temperature/top_p dropped), false forces plain sampling. It is applied once at client construction next to use_responses_api. The generated model editor shows it for openai and bedrock providers, since Bedrock Mantle also serves OpenAI-format models; it is ignored for Anthropic models on Bedrock.
• Remove the IsGPT6Astra transport fallback in chatopenai.UsesResponsesAPI; the bumped SDK recognizes gpt-6 itself (IsGPT6Astra stays for the effort clamp).
• Regenerated API docs, swagger, TypeScript types, and the model options schema; ARCHITECTURE.md documents the flag.

Validation

• New tests: TestChatModelCallConfig_ReasoningModelRoundTrip, TestModelFromConfig_OpenAIReasoningModelOverride (request body has reasoning and no temperature when forced on an unknown model; forced off on gpt-5 drops reasoning and keeps temperature; unset unchanged), TestAIGatewayModelBedrockReasoningModelOverride (Bedrock provider path), TestIsZeroChatModelCallConfigReasoningModel, and form-logic cases serializing the field for openai and bedrock. Red-green: removing the constructor wiring fails the request-body assertions.
• go test ./coderd/x/chatd/... -count=1 (full), go test ./codersdk/... -run ChatModel, go test ./coderd/ -run 'ChatModel|Chats', golangci-lint run ./coderd/x/chatd/... ./codersdk/... ./coderd/ (0 issues), make gen (stable on rerun), Vitest for src/modules/aiModels and src/api/chatModelOptions (174 tests), Biome, tsc --noEmit, make pre-commit.
• Remote dogfood UAT (Coder Agents) exercised the editor and chat on the dev AI gateway: the switch shows for OpenAI and Bedrock providers only, persists true/false/unset, and gpt-daybreak-blue-latest (an alias the SDK does not recognize) sends reasoning without temperature when forced on and gets OpenAI's temperature 400 when off or unset. UAT also found the new switch colliding with Use Responses API at 375px; tri-state switches now take the full row below sm. One open observation for follow-up: in Coder chats on gpt-6-astra the upstream stream carried no reasoning-summary events even though reasoning.summary=detailed was sent, while the same model returns summaries to a standalone fantasy client; not attributed to this change.
• The local pre-push hook fails in this checkout only on unrelated tests (umask-dependent TestGetModulesArchive, two untouched site tests), so the push bypassed it; CI is the gate.

Depends on coder/fantasy#57 merging; the pin then moves to the merged coder_2_33 commit. Related: #29229 (Bedrock reasoning_summary coercion).

Xum acted on behalf of @ibetitsmike for this PR.