feat: code improvements based on SonarCloud findings for the Api-Credential Resources

[diya-dhan]

Purpose

A few code changes have been introduced to optimise the code. These are based on suggestions given by the SonarCloud checks.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[x] Feature
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Test the code via automated test

go test ./...

What to Check

Verify that the following are valid:

• Automated tests are executed successfully

Other Information

Links to the issues reported by SonarCloud:

• Duplicate code in the api-credentials facade: https://sonarcloud.io/project/issues?impactSeverities=MEDIUM&issueStatuses=OPEN%2CCONFIRMED&id=SAP_terraform-provider-btp
• Repetition of the same literal: https://sonarcloud.io/project/issues?impactSeverities=HIGH&issueStatuses=OPEN%2CCONFIRMED&id=SAP_terraform-provider-btp

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• [ ] The PR is assigned to the Terraform project and a status is set (typically "in review").
• [ ] The PR has the matching labels assigned to it.
• [ ] The PR has a milestone assigned to it.
• [ ] If the PR closes an issue, the issue is referenced.
• [ ] Possible follow-up items are created and linked.