preserve_reasoning treats OPENAI_BASE_URL as a first-party OpenAI destination #4594

Description

pkg/model/provider/openai decides whether a destination is first-party OpenAI in Client.firstPartyOpenAIDestination, which reads ModelConfig.BaseURL. openai.NewClient resolves the effective endpoint differently — base_url, then OPENAI_BASE_URL, then https://api.openai.com/v1 — and stores that result on the embedded config (base.Config.BaseURL). The OpenAI Go SDK also honours OPENAI_BASE_URL on its own.

A model config that names no base_url but points the environment at a local server is therefore classified as first-party OpenAI.

Expected Behavior

provider_opts.preserve_reasoning should not replay encrypted OpenAI reasoning items to an endpoint that is not OpenAI. The first-party determination should consider the endpoint requests actually go to, the way reasoning replay now does in #4485 (Client.effectiveBaseURL).

Actual Behavior

With provider: openai, no base_url, and OPENAI_BASE_URL=http://localhost:8080/v1:

• firstPartyOpenAIDestination() returns true, so preservesResponseState() is on and reasoning items captured from OpenAI are replayed to the local server.
• shouldMergeConsecutiveMessages has the same blind spot — it also reads the config field — though that one only affects consecutive-message merging.

Reaching it takes an explicit preserve_reasoning: true, so the blast radius is narrow, but the effect is the opposite of what firstPartyResponses' own comment intends ("Opaque state must not leak to a different vendor's compatible endpoint").

Steps to Reproduce

1. Configure provider: openai with provider_opts: {preserve_reasoning: true} and no base_url.
2. Export OPENAI_BASE_URL=http://localhost:8080/v1 against llama.cpp or vLLM.
3. Run a turn that captures reasoning items, then inspect the next request body.

Additional context

Found while fixing #4363 in #4485. That PR deliberately leaves this path alone: it predates the PR, belongs to the Responses mechanism rather than Chat Completions, and changing it there would widen a review that is already about a different default. Happy to follow up with a PR.