llms/openai: openai client: return context.Canceled instead of new error declaration

[mats852]

Returning the standard context.Canceled error from llms/openai/internal/openaiclient/openaiclient.go instead of a new declaration, so callers can evaluate it down the line.

Moreover, since cancelled can be spelled with both 1 and 2 L, it is error prone. Go's standard declaration uses 1 L.

before

completion, err := llms.GenerateFromSinglePrompt(ctx, model, "prompt")

if err != nil && strings.Contains(err.Error(), "cancelled") {
	// handle canceled context
}

after

completion, err := llms.GenerateFromSinglePrompt(ctx, model, "prompt")

if errors.Is(err, context.Canceled) {
	// handle canceled context
}

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

[mats852]

To prevent breaking implementation, we could join errors also, let me know what you think.