Fix: Valid CCPA consent was dropped when gdpr_applies=true

[karwaankit32]

Fixing bug - #4867

[anishb-ms]

Both of these test cases in amp_auction_test.go set ConsentType: amp.ConsentUSPrivacy. The default: branch of ReadPolicy (unset consent_type, i.e. ConsentType == 0) is also affected by the fix but untested. Add a case that omits ConsentType (defaults to 0 → default branch):

        {
            desc: "amp.Params default consent_type with valid CCPA consent and gdpr_applies true - expect consent warning and remaining overrides applied",
            given: testInput{
                ampParams: amp.Params{
                    Consent:     "1YNN",
                    // ConsentType intentionally unset -> auto-detect (default branch)
                    GdprApplies: &gdprApplies,
                    Targeting:   `{"foo":"bar"}`,
                    Timeout:     &timeout,
                    Trace:       "verbose",
                },
                bidRequest: &openrtb2.BidRequest{
                    Imp: []openrtb2.Imp{{Banner: &openrtb2.Banner{Format: []openrtb2.Format{}}}},
                },
            },
            expected: testOutput{
                bidRequest: &openrtb2.BidRequest{
                    Imp: []openrtb2.Imp{{
                        Banner: &openrtb2.Banner{Format: []openrtb2.Format{}},
                        Ext:    json.RawMessage(`{"data":{"foo":"bar"}}`),
                    }},
                    Regs: &openrtb2.Regs{USPrivacy: "1YNN"},
                    Site: &openrtb2.Site{Ext: json.RawMessage(`{"amp":1}`)},
                    TMax: 500,
                    Ext:  json.RawMessage(`{"prebid":{"trace":"verbose"}}`),
                },
                errorMsgs: []string{"AMP request gdpr_applies value was ignored because provided consent string is a CCPA consent string"},
            },
        },

[karwaankit32]

added