fix(pluginhost): preserve HTTP status codes in host callback errors

[yontani]

Problem

Native host callback errors lose their numeric HTTP status when modelExecutionError returns a plain cause and when the Unix/Windows bridges serialize only err.Error(). A plugin receiving an opaque 503 cannot distinguish it from a terminal request error.

An outer StatusCode accessor can also return 0 while wrapping a valid 429/503. This occurs with the scheduler auth.WithCause/auth_unavailable wrapper, so stopping at the first errors.As match is insufficient.

Change

• Share a depth-first, pre-order error-tree traversal through Unwrap() error and Unwrap() []error, returning the first valid 400–599 status. Invalid outer accessors do not hide valid causes.
• Fall back to ErrorMessage.StatusCode, then 500 only when no valid status exists in a model execution error tree.
• Preserve messages and error identity through Unwrap, including errors.Is and concrete errors.As behavior.
• Serialize preserved statuses using the existing pluginabi.Error.HTTPStatus field on Unix and Windows. No C ABI/schema change; untyped non-model errors continue to omit status.

Review follow-up

Addressed discussion_r4021543669 in f107e67 with regressions using the real auth.WithCause wrapper and joined error trees.

Validation

Go 1.26.0, Linux ARM64, CGO enabled:

• Confirmed direct-status regression failure before the original fix.
• Confirmed zero-status auth_unavailable wrapper regression failure before the review follow-up, then success afterward.
• go test -p 2 -count=1 ./internal/pluginhost ./sdk/pluginabi ./sdk/api/handlers
• Server build and targeted race tests for all added test functions.
• Windows ARM64 test binary cross-compilation (CGO_ENABLED=0); not a Windows runtime test.
• gofmt and git diff --check.

Tests cover execute/stream-start envelopes, valid/invalid status boundaries, oversized integers, fallback precedence, nested and joined causes, cancellation identity and unchanged messages. The full repository suite was not run.

A release-based equivalent was exercised with the unchanged native fallback plugin and isolated synthetic upstreams: opaque503/429 fallback,400/422 stop,primary success,output-after-failure and cancellation-after-output. The deployment-specific harness is intentionally excluded from this minimal six-file PR.

[github-actions]

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The base branch has been automatically changed to dev.

[chatgpt-codex-connector]

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