refactor(adk): make the foreground handoff API usable outside the module

[hi-pender]

Why

Two related problems on the public surface around foreground/background handoff.

1. ShouldAutoBackground cannot be used outside this module.

Its parameter type is foreground.CandidateInfo, defined in adk/internal/foreground. Packages under adk/ may import it, so CI stayed green — but an external caller gets:

use of internal package github.com/cloudwego/eino/adk/internal/foreground not allowed

There is no workaround: Go does not infer function-literal parameter types from assignment context, and a defined type is not identical to a structurally equal struct literal, so no assignable closure can be written. The field was unusable for every user of the framework. It appeared on five public configs (backgroundtask/local.Config, backgroundtask/tool.ManagedToolConfig, middlewares/subagent, middlewares/filesystem, prebuilt/deep). The Config docs also referenced foreground.DefaultTimeoutMs, equally unreachable.

2. LocalBackgroundConfig.AlwaysForeground names the wrong thing.

Its only effect is selecting what the tool's timeout argument bounds — the caller's foreground wait, or the command's own execution limit passed to filesystem.ExecuteRequest.Timeout. "Always foreground" named a derived consequence rather than the thing being configured, and hid the cost: the mode disables the Manager's foreground timer, so context cancellation becomes the only bound on a backend that ignores Timeout.

Worth noting because it is easy to assume otherwise: a nil ShouldAutoBackground is not equivalent to this mode. On timer expiry it calls cancel() and returns a *backgroundtask.ForegroundTimeoutError rather than staying attached.

What

• Add backgroundtask.ForegroundCandidate (alias for the internal type) and backgroundtask.DefaultForegroundTimeoutMs. Same pattern as callbacks.RunInfo and schema.GobSerializer. foreground.Policy and ProjectionDetached stay internal — they are wiring, not user API.
• Point all five ShouldAutoBackground fields at the public name, and the internal construction sites too, so no file carries two names for one type.
• Rename AlwaysForeground → BackendOwnsCommandTimeout, with the doc comment restructured to state the timeout ownership, the cost, and the never-auto-backgrounded implication separately.
• Rename the description variants to BackendTimeoutManagedExecuteToolDesc / ...Chinese.
• Rename the internal helper to backendOwnsTimeoutForRun so it no longer collides with the struct field it reads.

No behavior change. Breaking change to API that has only shipped in v0.10.0-alpha.*.

Verification

• go build ./...
• go test -race ./... — full suite passes
• golangci-lint run ./adk/... — 0 issues
• gofmt -s -l ./adk — clean
• Confirmed from a separate module (replace to this tree) that both renamed APIs are now reachable: constructing the callback, reading every ForegroundCandidate field, and referencing DefaultForegroundTimeoutMs and BackendOwnsCommandTimeout all compile and run.

🤖 Generated with Claude Code

[codecov]

Codecov Report

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Additional details and impacted files

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