fix: resolve RealIP for bare RemoteAddr without port

[sonnemusk]

Summary

Context.RealIP() (and the LegacyIPExtractor fallback) used net.SplitHostPort and ignored the error, so a bare IP RemoteAddr (no :port) returned an empty string.

Change

• Fall back through extractIP, the same helper used by ExtractIPDirect.
• Add unit tests for bare IPv4 and IPv6 RemoteAddr values.

Test plan

• go test . -run TestContext_RealIP

[aldas]

In which environment request.RemoteAddr has only IP? When I am debugging with the actual http server - it has port?

[sonnemusk]

Good question — under a normal net/http TCP server, RemoteAddr is always host:port.

Where a bare IP shows up in practice:

1. Manually built / rewritten requests — unit tests, internal sub-requests, and some gateway adapters set RemoteAddr to just an IP (or an IPv6 address without brackets/port).
2. Consistency with ExtractIPDirect — that helper already falls back correctly when SplitHostPort fails (net.ParseIP). The default RealIP path (no IPExtractor) previously returned "" in the same situation, so callers that rely on the default got a different result than ExtractIPDirect.

So this is less “production TCP omits the port” and more “make the documented fallback robust and consistent with ExtractIPDirect”. Happy to narrow the PR description to that if you prefer.