False negative in go/email-injection for password reset links built from Forwarded / X-Forwarded-Host

Hi team,

I think I found a false negative in `go/email-injection`.

I ran into this while looking at ZITADEL's `CVE-2025-64101` / `GHSA-mwmh-7px9-4c23`. The vulnerable flow is:

- host data comes from `Forwarded` / `X-Forwarded-Host`
- it is stored in a request/domain context object
- that value is later carried through an event / notification path
- and eventually used to build a password reset link that gets emailed to the user

Very roughly, it looks like this:

```go
hostFromHeader = r.Header.Get(header)

if host == "" {
host = hostFromHeader
}

TriggeredAtOrigin: http.DomainContext(ctx).Origin()

url = login.InitPasswordLink(http_utils.DomainContext(ctx).Origin(), user.ID, code, user.ResourceOwner, authRequestID)
```

The upstream fix sanitizes the host before storing it in the domain context, so this seems like a real missed case, not just a noisy benchmark result.

My guess is that the source side is already covered well enough, since `net/http.Request.Header` is modeled as remote input. The gap seems to be later in the flow, once the value moves through custom structs / event fields / notification helpers before it becomes part of email content.

I think this pattern is fairly common in real Go code. Password reset and verification links are often built indirectly through app-specific context and mailer abstractions, not directly inside a modeled mail API call.

For comparison, Semgrep did flag this codebase, but only with a broader rule about request-derived origin/host usage. It was not precise, but the general idea may still be useful here: request-header-derived host/origin values that later become user-facing links in emails probably deserve better coverage.

A reasonable fix might be improving `go/email-injection` so flow is preserved better through custom carrier objects and “build link first, send email later” patterns.

Start at go/email-injection and trace how its source-to-sink flow handles values moved through custom structs, event fields, and notification helpers. Compare this with the described Forwarded/X-Forwarded-Host to password-reset-link path; done means the query covers the indirect flow and avoids the reported false negative.

索引モデルが issue の本文から書いたものです。