[main -> 1.0.x] Backport OAuth fixes

[thiva-k]

Purpose

Backport three OAuth fixes from main to 1.0.x.

Approach

Cherry-picked unchanged; diffs are patch-identical to main:

• Restrict token introspection to access and refresh tokens #4984 (739b7bcc1) - Restrict token introspection to access and refresh tokens
• Bind CIBA-issued access tokens to the DPoP proof key #4983 (4c4323cb2) - Bind CIBA-issued access tokens to the DPoP proof key
• Require id_token_hint to be an ID token at the logout endpoint #4991 (721e49afb) - Require id_token_hint to be an ID token at the logout endpoint

Related Issues

• N/A

Related PRs

• Restrict token introspection to access and refresh tokens #4984
• Bind CIBA-issued access tokens to the DPoP proof key #4983
• Require id_token_hint to be an ID token at the logout endpoint #4991

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[codecov]

Codecov Report

❌ Patch coverage is 96.07843% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ackend/internal/oauth/oauth2/introspect/service.go	88.88%	1 Missing and 1 partial ⚠️

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