FINERACT-2718: Migrate group center tests to feign

[DeathGun44]

What this PR does

Migrates the group and center integration tests from the old REST-assured HTTP client to the typed Feign SDK client

Why

The old tests call the server with hand-written REST-assured requests and read untyped JSON. Moving them onto the generated Feign client gives typed requests and responses, so the tests are safer to change and consistent with the rest of the migrated suite.

What changed

Test classes migrated

• CenterIntegrationTest
• GroupTest
• GroupLoanIntegrationTest

New typed helpers:

• FeignGroupHelper, FeignCenterHelper, FeignStaffHelper, FeignGlimHelper

Removed the old stopgap:

• Deleted FeignGroupCenterHelper (a raw-HTTP helper from an earlier PR) and repointed its two users LoanReschedulingWithinCenterTest and ExternalIdSupportIntegrationTest - at the new typed helpers.

Production change (minimal, additive):

• Added the missing fields to PostGroupsRequest and PostCentersRequest in the Swagger DTOs so groups and centers can be created through the typed model. These are additive only - the API wire format is unchanged.

Where raw HTTP is still used (and why)

A few calls still go through FeignRawHttpHelper (plain HttpURLConnection, not REST-assured) - only where the generated models genuinely can't express what the test asserts:

• group active / clientMembers and center fields the response models don't expose,
• shared ?command=... endpoints whose request models don't carry the command fields,
• the GLIM application / approve / retrieval flow (multi-child; returns fields the SDK doesn't model).

Each is documented at the call site. No assertion was weakened and no REST-assured
was introduced.

Checklist

Please make sure these boxes are checked before submitting your pull request - thanks!

• Write the commit message as per our guidelines
• Acknowledge that we will not review PRs that are not passing the build ("green") - it is your responsibility to get a proposed PR to pass the build, not primarily the project's maintainers.
• Create/update unit or integration tests for verifying the changes made.
• Follow our coding conventions.
• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes
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• If merging this PR resolves a JIRA issue, I will mark that issue as resolved and set "Fix Version/s" appropriately.

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