Bug: [REFACTOR] Unify the `sandbox` and `production` cargo features



### Description

After #1349, this is the next step towards using a single Docker image across all our environments (see #1346). The `sandbox` and `production` features would have to be unified into a single `release` feature instead. This change would also need corresponding changes in the `Dockerfile` and CI workflows.
diff --git a/Dockerfile b/Dockerfile
index 2805ee18325..fb92bd3acb5 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,6 +1,5 @@
 FROM rust:slim as builder
 
-ARG RUN_ENV=sandbox
 ARG EXTRA_FEATURES=""
 
 RUN apt-get update \
@@ -33,7 +32,7 @@ ENV RUST_BACKTRACE="short"
 ENV CARGO_REGISTRIES_CRATES_IO_PROTOCOL="sparse"
 
 COPY . .
-RUN cargo build --release --features ${RUN_ENV} ${EXTRA_FEATURES}
+RUN cargo build --release --features release ${EXTRA_FEATURES}
 
 
 
diff --git a/crates/router/Cargo.toml b/crates/router/Cargo.toml
index 254503f57d4..4f70c0a7f44 100644
--- a/crates/router/Cargo.toml
+++ b/crates/router/Cargo.toml
@@ -16,8 +16,7 @@ kms = ["external_services/kms","dep:aws-config"]
 email = ["external_services/email","dep:aws-config"]
 basilisk = ["kms"]
 stripe = ["dep:serde_qs"]
-sandbox = ["kms", "stripe", "basilisk", "s3", "email"]
-production = ["kms", "stripe", "basilisk", "s3", "email"]
+release = ["kms", "stripe", "basilisk", "s3", "email"]
 olap = []
 oltp = []
 kv_store = []
diff --git a/crates/router/src/core/payment_methods/transformers.rs b/crates/router/src/core/payment_methods/transformers.rs
index e65ee980c0e..69fa59f906b 100644
--- a/crates/router/src/core/payment_methods/transformers.rs
+++ b/crates/router/src/core/payment_methods/transformers.rs
@@ -241,11 +241,7 @@ pub async fn mk_add_card_request_hs(
     customer_id: &str,
     merchant_id: &str,
 ) -> CustomResult<services::Request, errors::VaultError> {
-    let merchant_customer_id = if cfg!(feature = "sandbox") {
-        format!("{customer_id}::{merchant_id}")
-    } else {
-        customer_id.to_owned()
-    };
+    let merchant_customer_id = customer_id.to_owned();
     let card = Card {
         card_number: card.card_number.to_owned(),
         name_on_card: card.card_holder_name.to_owned(),
@@ -359,7 +355,7 @@ pub fn mk_add_card_request(
     locker_id: &str,
     merchant_id: &str,
 ) -> CustomResult<services::Request, errors::VaultError> {
-    let customer_id = if cfg!(feature = "sandbox") {
+    let customer_id = if cfg!(feature = "release") {
         format!("{customer_id}::{merchant_id}")
     } else {
         customer_id.to_owned()
@@ -398,11 +394,7 @@ pub async fn mk_get_card_request_hs(
     merchant_id: &str,
     card_reference: &str,
 ) -> CustomResult<services::Request, errors::VaultError> {
-    let merchant_customer_id = if cfg!(feature = "sandbox") {
-        format!("{customer_id}::{merchant_id}")
-    } else {
-        customer_id.to_owned()
-    };
+    let merchant_customer_id = customer_id.to_owned();
     let card_req_body = CardReqBody {
         merchant_id,
         merchant_customer_id,
@@ -482,11 +474,7 @@ pub async fn mk_delete_card_request_hs(
     merchant_id: &str,
     card_reference: &str,
 ) -> CustomResult<services::Request, errors::VaultError> {
-    let merchant_customer_id = if cfg!(feature = "sandbox") {
-        format!("{customer_id}::{merchant_id}")
-    } else {
-        customer_id.to_owned()
-    };
+    let merchant_customer_id = customer_id.to_owned();
     let card_req_body = CardReqBody {
         merchant_id,
         merchant_customer_id,

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR unifies the `sandbox` and `production` cargo features into a single feature named `release`.

## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless its an obvious bug or documentation fix
that will have little conversation).
-->
Closes #1353.

## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->
Built a Docker image successfully using the command `docker build --build-arg BINARY=router --build-arg RUN_ENV=sandbox .`.

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable
