backend/http: Refresh state earlier (align with other backends)

[radeksimko]

This updates the http backend's behaviour such that it refreshes state prior to returning the StateMgr. In practice this should not have any impact on the end user, perhaps just that errors will be raised a little bit earlier.

This change however enables us to now use that backend to test edge cases and prevent bugs such as #38027

Prior to this patch, the http backend is not actually impacted by the crash reported in #38027

Some backends (azure, gcs, kubernetes) are assumed to be impacted as they implement the exact same behaviour I'm trying to mimic here:

terraform/internal/backend/remote-state/azure/backend_state.go

Lines 107 to 110 in 193867f

	// Grab the value
	if err := stateMgr.RefreshState(); err != nil {
	return nil, diags.Append(err)
	}

terraform/internal/backend/remote-state/gcs/backend_state.go

Lines 108 to 111 in 193867f

	// Grab the value
	if err := st.RefreshState(); err != nil {
	return nil, diags.Append(err)
	}

terraform/internal/backend/remote-state/kubernetes/backend_state.go

Lines 94 to 97 in 193867f

	// Grab the value
	if err := stateMgr.RefreshState(); err != nil {
	return nil, diags.Append(err)
	}

Other backends implement the same logic conditionally (only refresh once a previously not-existing state was auto-created) but the bug likely impacts them in a similar way anyway:

terraform/internal/backend/remote-state/consul/backend_state.go

Lines 120 to 124 in 193867f

	// Grab the value
	if err := stateMgr.RefreshState(); err != nil {
	err = lockUnlock(err)
	return nil, diags.Append(err)
	}

terraform/internal/backend/remote-state/cos/backend_state.go

Lines 126 to 130 in 193867f

	// Grab the value
	if err := stateMgr.RefreshState(); err != nil {
	err = lockUnlock(err)
	return nil, diags.Append(err)
	}

terraform/internal/backend/remote-state/oci/backend_state.go

Lines 88 to 91 in 193867f

	if err := stateMgr.RefreshState(); err != nil {
	err = lockUnlock(err)
	return nil, diags.Append(err)
	}

terraform/internal/backend/remote-state/oss/backend_state.go

Lines 162 to 166 in 193867f

	// Grab the value
	if err := stateMgr.RefreshState(); err != nil {
	err = lockUnlock(err)
	return nil, diags.Append(err)
	}

terraform/internal/backend/remote-state/s3/backend_state.go

Lines 231 to 234 in 193867f

	if err := stateMgr.RefreshState(); err != nil {
	err = lockUnlock(err)
	return nil, diags.Append(err)
	}

---

I'm open to breaking away the last commit and reviewing it alongside #38028 - we cannot really merge this PR on its own without #38028 anyway but having it crash like this in isolation hopefully demonstrates the value.

[SarahFrench]

Thanks, LGTM!