feat(anthropic,fireworks,openai): use LangSmith gateway keys

[Mukil Loganathan (langchain-infra)]

Description

When LANGSMITH_GATEWAY is enabled, OpenAI, Anthropic, and Fireworks chat models now resolve gateway auth from LANGSMITH_GATEWAY_API_KEY, LANGSMITH_API_KEY, then LANGCHAIN_API_KEY, while preserving explicit constructor keys.

Release Note

LangSmith Gateway mode can authenticate with existing LangSmith API key environment variables.

Test Plan

• langsmith_gateway unit tests for OpenAI, Anthropic, and Fireworks

Made by Open SWE

[open-swe]

Open SWE Review found 2 potential issues.

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[open-swe]

🟡 Empty key masks valid gateway fallbacks

secret_from_env stops at the first variable that exists, even when its value is empty. Thus with gateway mode enabled and LANGSMITH_GATEWAY_API_KEY="" plus a valid LANGSMITH_API_KEY, this returns SecretStr("") instead of trying the documented fallback. OpenAI then replaces even a valid OPENAI_API_KEY with the empty secret; the identical helpers in Anthropic (line 98) and Fireworks (line 130) respectively fall through to the provider key or raise for a missing provider key rather than using the valid lower-priority gateway key. This makes authentication fail in environments that define the higher-priority variable as empty, despite a valid fallback being configured.

(Refers to lines 198-199)

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[open-swe]

🟡 Explicit None disables gateway authentication

The public api_key type permits None, but field-set membership treats ChatOpenAI(..., api_key=None) as an explicit credential. In gateway mode this skips _resolve_gateway_api_key(), leaves both clients without authentication, and invocation fails even when LANGSMITH_GATEWAY_API_KEY is set. Before this change, a None value entered the gateway fallback; this matters for callers that forward an optional configuration value. Only a non-None explicitly supplied key should suppress environment resolution.

(Refers to lines 1224-1225)

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Suggested change

	explicit_api_key = bool({"api_key", "openai_api_key"} & self.model_fields_set)
	if _base_url_from_gateway and not explicit_api_key:
	explicit_api_key = self.openai_api_key is not None and bool(
	{"api_key", "openai_api_key"} & self.model_fields_set
	)
	if _base_url_from_gateway and not explicit_api_key: