fix: append /api/v3 and /api/graphql paths for GHEC Data Residency hosts

[JiayangZhou]

Description

This PR fixes a regression introduced in v6.6.0 where GHEC Data Residency hosts (*.ghe.com) were incorrectly treated like github.com, resulting in the API being unreachable.

Problem

In PR #2547, support for GHEC Data Residency was added with the assumption that *.ghe.com hosts behave like github.com (i.e., they don't need /api/v3 or /api/graphql/ paths). However, this assumption is incorrect - these hosts actually behave like GitHub Enterprise Server and do require the API paths.

Symptoms

• With v6.5.0: Users configured base_url = "https://customer.ghe.com/api/v3" and it worked
• With v6.6.0: Validation requires removing /api/v3, but then the provider fails to reach the API
• Error: this resource can only be used in the context of an organization, "ORG_NAME" is a user

Solution

Remove the GHECDataResidencyHostMatch exclusion from the path construction logic in:

• config.go: NewGraphQLClient() and NewRESTClient()
• apps.go: getInstallationAccessToken()

This ensures all non-api.github.com hosts (including *.ghe.com) will have the correct API paths appended.

Testing

• [x] All existing tests pass
• [x] Manually tested with a real GHEC Data Residency environment (hidden-staging.ghe.com)
• [x] terraform plan successfully refreshes all GitHub resources

Related Issues

Fixes the issue reported in the diff comparison: https://github.com/integrations/terraform-provider-github/compare/v6.5.0...v6.6.0#diff-062b7a475c9ddd7765fd96c620a3f7854307a4b68c0ff57db24b9d912b5bcec9R88

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