Add IAM service HTTP client and OAuth consent flow services

What does this MR do and why?

Extract shared HTTP infrastructure for IAM service integrations and introduce the service layer for the OAuth consent flow.

• Introduce HttpClient to encapsulate URL construction, IAM auth headers, timeout, and transport error handling
• Introduce RedirectUrlValidator to validate IAM redirect URLs
• Refactor AcceptLoginChallengeService to use HttpClient
• Add GetConsentChallengeService, AcceptConsentChallengeService, and RejectConsentChallengeService for the consent flow
• Log HTTP status codes only on failure; response body logging removed in favour of a more structured error.

The controller wiring these services into the consent flow will be introduced in a follow-up MR.

References

• Main issue: #589572
• AcceptLoginChallengeService was introduced by: !225853 (merged)

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