Extract shared Lark outbound retry helper (DRY TrySendWithFallbackAsync between SkillRunner and FeishuCardHumanInteractionPort)

[eanzhao]

Context

Surfaced as architecture observation #1 in PR #412 review.

Problem

PR #412 added near-identical TrySendWithFallbackAsync + SendOutcome record + SendOutboundAsync in two places:

• SkillRunnerGAgent.cs
• FeishuCardHumanInteractionPort.cs

FeishuCardHumanInteractionPort's implementation has a comment that explicitly says "Mirrors SkillRunnerGAgent.TrySendWithFallbackAsync". Already drifting:

• Different log strings: Skill runner ... primary delivery target rejected as ... vs Feishu human interaction port primary delivery target rejected as ...
• SendOutcome record duplicated as a private nested type in each class

This violates CLAUDE.md "删除优先：空转发、重复抽象、无业务价值代码直接删除". Future retry-policy tweaks (additional error codes, backoff, telemetry) require touching both copies and they will keep drifting.

Proposal

Extract a shared retry helper that takes the send delegate plus the primary/fallback target pair:

internal static class LarkOutboundRetryPolicy
{
    public static async Task<SendOutcome> SendWithFallbackAsync(
        Func<LarkReceiveTarget, CancellationToken, Task<string>> sendAsync,
        LarkReceiveTarget primary,
        LarkReceiveTarget? fallback,
        ILogger logger,
        string callerContext,
        CancellationToken ct)
    {
        // shared retry-on-230002 policy here
    }
}

Each callsite passes its own sendAsync delegate (different proxy paths, different headers) and a callerContext string for logs. The retry policy itself lives in one place.

Acceptance:

• Single implementation of the retry policy (one SendOutcome type, one log line shape, one error-code allowlist).
• SkillRunnerGAgent.SendOutputAsync and FeishuCardHumanInteractionPort.SendMessageAsync both call the shared helper.
• Existing tests at both call sites continue to pass without test-side duplication.
• Adding a new error code to the retry allowlist (e.g. a third Lark bot_kicked variant) requires editing exactly one file.

Out of scope

• Moving the retry policy into a different DI-resolved abstraction (ILarkOutboundDispatcher) is tracked separately as the third architecture follow-up — that's a deeper boundary change. This issue is the minimum-viable DRY-up.