feat: surface UNPROCESSABLE_ENTITY reason when invite fails

[Hayena]

Summary

Implements #21

OrganizationService.invite() previously returned NotFound for both HTTP 404 and 422, silently losing the reason an invitation failed (e.g. user is already a member, validation error). This PR separates the two cases so callers get meaningful feedback.

Changes

• GitHubOrganizationClient.invite() now handles UNPROCESSABLE_ENTITY (422) separately from NOT_FOUND (404)
• 422 → ApiResult.Failure with the error message extracted from the response body
• 404 → ApiResult.NotFound (unchanged)
• GitHubApiException.from() updated to extract the message field from JSON response bodies
• KDoc for invite() updated to document all outcomes
• Unit tests added for the 422 case

Before / After

// Before — 422 was indistinguishable from 404
when (client.organizations.invite(org, "octocat")) {
    is ApiResult.Found    -> println("Invited")
    is ApiResult.NotFound -> println("Not found or validation failed") // ambiguous
    is ApiResult.Failure  -> println("Error")
}

// After — reason is explicit
when (client.organizations.invite(org, "octocat")) {
    is ApiResult.Found    -> println("Invited")
    is ApiResult.NotFound -> println("Organisation not found")
    is ApiResult.Failure  -> println("Failed: ${result.exception.message}") // e.g. "Validation Failed: already a member"
}