Fix custom IMDS endpoint path construction

[Copilot]

AZURE_POD_IDENTITY_AUTHORITY_HOST produced IMDS URLs containing //metadata/..., causing resource-not-found responses.

• Endpoint construction

  • Append the already-prefixed token path directly:

    builder.append(IMDS_TOKEN_PATH);

• Regression coverage

  • Assert custom authority hosts produce /metadata/identity/oauth2/token without a double slash.

• Fixes [Bug] AZURE_POD_IDENTITY_AUTHORITY_HOST env not used correctly #1037

[azure-pipelines]

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[Gladwin Johnson (gladjohn)]

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[azure-pipelines]

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