Support URL requests through active core outbounds

[eliotcougar]

Summary

• add CoreController.GetUrlContent for bounded text requests through the running Xray instance
• require an explicit outbound tag and force every dial through that outbound
• expose the first viable principle target of a running balancer
• add CoreController.DownloadUrlToFile for header-aware, binary-safe streaming to disk
• make file downloads transactional so incomplete responses cannot replace an existing destination
• keep the existing delay and lifecycle paths unchanged

Why

Android callers sometimes need to reuse the already-running Xray connection when no local proxy listener exists. The initial use is v2rayNG's current-connection test in native Xray TUN mode, where the configured connection-info URL supplies the displayed country code and IP address.

Calling core.Dial without selecting an outbound is insufficient: normal routing rules may send an app-initiated request through direct or another detour. The result would then describe the wrong connection. Large subscriptions and binary resource files also should not be returned as strings or passed through Binder.

Downstream use

v2rayNG #6109 uses these APIs to:

• fetch connection information through the active outbound
• download subscriptions through the running core when native Xray TUN is active and the local proxy is disabled
• stream geodata and other resource files through the same connection
• resolve balancer-main to a concrete member for primary policy groups

Ordinary profiles pass the proxy outbound. Policy groups use the first non-empty principle target exposed by Xray's existing routing feature. Strategies with several principle candidates remain best-effort; querying the target does not advance random or round-robin state.

The file-download API is intentionally general and can be reused by other Android callers that need subscriptions or resources to follow an existing Xray-core connection.

Implementation

The shared request helper snapshots the running core instance, applies session.SetForcedOutboundTagToContext inside DialContext, and opens connections with core.Dial. Redirects therefore remain on the requested outbound.

DownloadUrlToFile accepts request headers and a caller-defined timeout. It streams the response to a mode-0600 sibling temporary file, verifies a declared Content-Length, closes the file, and then replaces the destination with os.Rename. Read, write, close, or rename failures remove only the temporary file, preserving any existing destination.

Unsolicited HTTP 206 responses are rejected because the helper never sends a Range request. Other successful 2xx responses remain supported.

The implementation is confined to libv2ray_utils.go; libv2ray_main.go is unchanged.

Validation

• go test ./...
• four-ABI gomobile bind AAR build: armeabi-v7a, arm64-v8a, x86, and x86_64
• generated AAR API and ABI inspection
• downstream v2rayNG Play Store debug Kotlin compilation and focused HttpUtilTest

[eliotcougar]

Follow-up correction in 16b52e1: core.Dial uses Xray's normal dispatcher, so the original one-argument method still allowed routing rules to select direct or another detour. GetUrlContent now requires an outbound tag and applies Xray's forcedOutboundTag session metadata before every dial. The downstream v2rayNG caller passes its proxy tag, ensuring the country/IP result represents the selected proxy even when the local proxy listener is disabled.

This explicit contract also makes the API suitable for future subscription and resource-file downloads through the running core: those callers can select the intended outbound instead of inheriting user routing accidentally.