provider now checks for FieldSetNamespaceFromToken being true before prepending token namespace

[johnjosephg]

Description

The provider now makes sure that set_namespace_from_token is set to true before prepending the namespace of the token to the request, honoring the setting without disturbing existing functionality.

Checklist

• Added CHANGELOG entry (only for user-facing changes)
• Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

go test -v ./vault -run 'TestAccNamespaceProviderConfigure_setNamespaceFromToken' -count=1
=== RUN   TestAccNamespaceProviderConfigure_setNamespaceFromToken
2026/06/25 15:28:53 [INFO] Using Vault token with the following policies: root
=== RUN   TestAccNamespaceProviderConfigure_setNamespaceFromToken/false_does_not_prepend_token_namespace
=== RUN   TestAccNamespaceProviderConfigure_setNamespaceFromToken/true_prepends_token_namespace
--- PASS: TestAccNamespaceProviderConfigure_setNamespaceFromToken (1.87s)
    --- PASS: TestAccNamespaceProviderConfigure_setNamespaceFromToken/false_does_not_prepend_token_namespace (1.07s)
    --- PASS: TestAccNamespaceProviderConfigure_setNamespaceFromToken/true_prepends_token_namespace (0.79s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     2.908s

Output from exisitng unit tests for GetNSClient:

$ go test -v ./internal/provider/ -run 'TestProviderMeta_GetNSClient' -count=1

=== RUN   TestProviderMeta_GetNSClient
=== RUN   TestProviderMeta_GetNSClient/no-resource-data
=== RUN   TestProviderMeta_GetNSClient/basic-no-root-ns
=== RUN   TestProviderMeta_GetNSClient/basic-root-ns
=== RUN   TestProviderMeta_GetNSClient/basic-root-ns-trimmed
--- PASS: TestProviderMeta_GetNSClient (0.00s)
    --- PASS: TestProviderMeta_GetNSClient/no-resource-data (0.00s)
    --- PASS: TestProviderMeta_GetNSClient/basic-no-root-ns (0.00s)
    --- PASS: TestProviderMeta_GetNSClient/basic-root-ns (0.00s)
    --- PASS: TestProviderMeta_GetNSClient/basic-root-ns-trimmed (0.00s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/internal/provider 0.487s

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PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

[vijayavelsekar]

LGTM