fix(xhttp): don't append trailing slash to file-like paths

[voltara13]

Backport of the NormalizedPath fix onto v1.19.27. A trailing slash was forced onto every xhttp path, breaking paths that point at a file (e.g. /foo.txt). Skip the trailing slash when the last path segment contains a dot.

[wwqgtxx]

The current behavior matches that of the official client, and we have no interest in making any changes before XTLS/Xray-core#6307 is merged upstream.

[tuefalek]

The argument "matches the official client" doesn't hold here: the official client (Xray-core) has the same bug — XTLS/Xray-core#6307 is open precisely because GetNormalizedPath unconditionally appends a trailing slash, which is wrong when sessionID and seq are placed in query/cookie/header rather than in the path.
The concrete breakage: when using xhttp behind a CDN (e.g. Yandex CDN, Cloudflare), the CDN whitelist contains specific paths like /stream/data.bin. With the forced trailing slash, mihomo sends GET /stream/data.bin/ — a 404 or a cache miss, because CDN path matching is exact. The connection never establishes. This is not a theoretical concern; it affects any deployment that uses file-like paths for camouflage.
The fix in this PR is conservative: it skips the trailing slash only when the last path segment contains a dot. It does not change behavior for the default case (path-based sessionID/seq placement).
Blocking a correct fix in mihomo while waiting for upstream to merge a separate PR leaves users with a client that simply doesn't work in a common CDN scenario. Consider merging this as a stopgap, or at minimum reopening the PR when #6307 lands upstream.

[ashdkv]

The heuristic used in this PR (skip trailing slash when the last segment contains a dot) is a workaround, not the correct fix.

The trailing slash exists for one reason only: to separate the configured base path from the appended sessionID/seq segments, so the server can slice them back off via req.URL.Path[len(path):]. It is meaningful only when at least one of sessionID or seq uses path placement.

The correct condition is:

if sessionIDPlacement == "path" || seqPlacement == "path" {
    // append trailing slash
}

This is exactly what XTLS/Xray-core#6307 implements. If neither field goes into the path, the configured path must be sent verbatim — no slash appended, no matter what the path looks like. A dot-based heuristic fails for paths like /stream/v2 (no dot, but both fields in query) and is unnecessarily fragile.

[wwqgtxx]

It is pointless to spend time discussing implementation details with me; if you want to resolve your issue, please urge the upstream project to make the necessary changes. We will synchronize the updates as soon as they are merged upstream.

[Medium1992]

It is pointless to spend time discussing implementation details with me; if you want to resolve your issue, please urge the upstream project to make the necessary changes. We will synchronize the updates as soon as they are merged upstream.

Xray-core has merged the upstream fix:
XTLS/Xray-core@1aabe7e

Could this PR be reopened and adjusted to match Xray’s behavior?

The current PR uses a file-like path heuristic, while upstream now appends the trailing slash only when sessionID or seq are placed in the path.

Maybe mihomo could use the same condition as Xray here:

if c.GetNormalizedSessionPlacement() == PlacementPath ||
	c.GetNormalizedSeqPlacement() == PlacementPath {
	if !strings.HasSuffix(path, "/") {
		path += "/"
	}
}

[wwqgtxx]

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