fix(gofr): warn when readiness checks are registered but the HTTP server never starts

[ikio-nen]

Closes #4259.

Problem

An app that registers readiness checks but no HTTP routes never starts its HTTP server — silently:

app := gofr.New()
app.AddReadinessCheck(func(ctx *gofr.Context) error { return sqlPing(ctx) })
app.Run() // starts metrics only; /.well-known/health is never served

Run() gates startHTTPServer on httpRegistered, which is set only by route registration (GET/POST/GraphQL/CRUD) or the presence of a public/ static directory. With no routes and no public/, no port is bound, /.well-known/health is never served, and the startup log shows a perfectly healthy metrics server — while every readiness probe gets connection refused. Nothing says why.

The check-only app is unusual, but the failure is invisible when it happens: the metrics-port line in the log looks exactly like a normal startup.

Fix

startHTTPServer now logs a warning when it detects exactly that mismatch — readiness checks registered, HTTP server not marked for startup:

readiness: 1 app check(s) registered, but the HTTP server will not start — no route was registered and no public/ directory exists, so /.well-known/health is unreachable. Register at least one HTTP route.

The wording mirrors the existing readiness: N app check(s) registered info line, so both ends of the story read the same. Apps without readiness checks are unaffected and stay silent, so normal no-HTTP usage (pure metrics, gRPC-only, CMD) sees no new output.

Behavior change

None beyond the new WARN line: no API change, no config change, no startup-time cost for apps without readiness checks (the check is one nil-slice length read on a path that previously did nothing).

Verification

• New test TestApp_startHTTPServer_warnsWhenReadinessChecksCannotBeServed drives the real startHTTPServer: warns with checks + no routes, stays silent with neither. It pins httpRegistered explicitly because New() marks the server for startup whenever the working directory happens to contain a static directory — a workspace-dependent test would be flaky.
• End-to-end on a real app (register one readiness check, no routes): warning present in the startup log; with the /ping route added back, the warning disappears and health serves normally.
• go vet ./pkg/gofr/ clean; gofmt clean.

Checklist

• Formatted with gofmt (golangci-lint runs in CI).
• New code covered by unit tests.
• No decrease in overall code coverage.
• Code comments reviewed for clarity.