Bug: feat(cypress): `connectorUpdateCall` fails due to fixtures overwrite



The `connectorUpdateCall` fails in the Cypress Tests for connectors for which a certain metadata field is `required` (for e.g., `Braintree`) because the `cypress-tests/cypress/fixtures/update-connector-body.json` overwrites the Request Body.
## Type of Change
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- [ ] Bugfix
- [ ] New feature
- [x] Enhancement
- [x] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [x] CI/CD

## Description
<!-- Describe your changes in detail -->
The Connector Update Body is being overwritten, which included the metadata as well. Due to this, if there were any required metadata fields of a connector, then the Connector Update Test would fail.

For example, `Braintree` connector has a required metadata field, and while testing the Connector Update, just because it was being overwritten by the `update-connector-body.json` fixture, the test fails as shown in the following screenshot.
<img width="2117" alt="image" src="https://github.com/user-attachments/assets/bf8b384a-92be-4f86-bf09-48fd52f01db4" />

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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Provide links to the files with corresponding changes.

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2. `crates/router/src/configs`
3. `loadtest/config`
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## Motivation and Context
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Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

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While adding the Card test Flows for the `Braintree` connector, the Connector Update Test Failed.

## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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Running the `0000-CoreFlows.cy.js` for `Braintree` connector

<img width="2116" alt="image" src="https://github.com/user-attachments/assets/7b505791-4da3-4fae-a94c-142dcfc69b53" />

<img width="752" alt="image" src="https://github.com/user-attachments/assets/f9fc5350-4bf9-4b2e-a5da-a62264f5c1b1" />


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

- [ ] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [ ] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
