Add typed AuthError variants once the runtime distinguishes auth-missing/invalid/unreachable

这项工作受 github/copilot-agent-runtime#7485 阻塞；先阅读该 runtime issue，并定位 SDK 现有的 Error/Rpc 映射，因为这里没有指定 SDK 文件。存在不同的 wire code 后，将它们映射到 AuthErrorKind，并将未知 code 保留为 Rpc，然后验证 src-tauri/src/error.rs:115-120 和 handlers/auth.rs::emit_copilot_cli_auth_issue 不再需要字符串匹配或 connectivity workaround。

Context

Companion to github/copilot-agent-runtime#7485 — the runtime today returns identical generic -32603 internal_error payloads for "no token," "invalid token," and "network unreachable." Once the runtime adds distinct codes (e.g., -32001 AuthMissing, -32002 AuthInvalid, -32003 AuthUnreachable), the SDK should expose them as a typed enum so consumers don't string-match.

Proposed shape

pub enum Error {
    Rpc { code: i32, message: String, data: Option<Value> },
    // ...
    Auth(AuthErrorKind),  // new
}

pub enum AuthErrorKind {
    /// No token configured. User needs to sign in.
    Missing,
    /// Token rejected by CAPI (401/403). User needs to reauth.
    Invalid,
    /// Transport failure talking to CAPI. Retry, don't reauth.
    Unreachable { cause: String },
}

The SDK can map known runtime codes to Error::Auth(...) and leave anything else as Error::Rpc { ... }.

What this lets consumers delete

In Copilot Desktop today (src-tauri/src/error.rs:115-120) we string-match three magic phrases:

pub fn is_copilot_auth_failure(&self) -> bool {
    let message = self.to_string().to_ascii_lowercase();
    message.contains("not authenticated")
        || message.contains("authenticate first")
        || message.contains("no authentication info available")
}

Plus the connectivity-tracker workaround in handlers/auth.rs::emit_copilot_cli_auth_issue whose only job is to second-guess whether the runtime's "not authenticated" really means "not authenticated" or "couldn't reach the auth service." Both go away with typed variants.

Dependencies

Blocked on the runtime change (github/copilot-agent-runtime#7485). No useful SDK work to land before the runtime distinguishes the cases on the wire.

  _{Generated via Copilot (Claude Opus 4.7) on behalf of @tclem}