feat(featureflag): guarantee non-nil HTTP transport

Summary

Guarantees that the *http.Client passed to the Flipt provider in v2/featureflag always has a non-nil Transport, while preserving any caller-supplied transport. Prevents panics in downstream callers that wrap or introspect the transport — previously the field was left nil and relied on the implicit stdlib fallback to http.DefaultTransport.

Behaviour

A new internal helper resolveHTTPClient is the single point of responsibility for HTTP client resolution in NewWithConfig:

Caller input	Resulting client passed to Flipt
cfg.HTTPClient == nil	New &http.Client{Timeout: defaultHTTPClientTimeout, Transport: http.DefaultTransport}
cfg.HTTPClient != nil, Transport != nil	Used as-is, preserved by identity
cfg.HTTPClient != nil, Transport == nil	Shallow-cloned with Transport = http.DefaultTransport. Caller's struct is not mutated.

The shallow-clone branch (clone := *cfg.HTTPClient) copies Timeout, Jar, and CheckRedirect verbatim and is the reason a caller-supplied client with a nil transport doesn't have its struct mutated as a side effect.

What's in this MR

• v2/featureflag/client.go — new resolveHTTPClient helper; NewWithConfig now calls it; Config.HTTPClient doc comment documents the guarantee.
• v2/featureflag/transport_test.go — three unit tests (nil input, explicit transport preserved, nil transport cloned without mutation), in package featureflag so they can call resolveHTTPClient directly.

Test plan

• go test ./featureflag/... passes locally (full v2 module also green)
• golangci-lint run ./featureflag/... reports 0 issues
• Three new unit tests cover the three input cases enumerated above

Notes for reviewers

• The default transport is http.DefaultTransport — the same value the stdlib falls back to implicitly. This MR makes the existing fallback explicit; transport tuning (MaxIdleConns, IdleConnTimeout, etc.) is intentionally out of scope.
• resolveHTTPClient stays unexported. The transport tests are in package featureflag (white-box) rather than package featureflag_test, so no export shim is needed.