Clarify credential request client definition to support external implementations

[Copilot]

This PR addresses concerns raised by Tim about the restrictive language in the "credential request client" definition that could be interpreted as requiring the client to always be a user agent-owned component.

Problem

The original definition used "user-agent-defined component" language, which could exclude real-world deployments where:

• The credential selector/client is implemented by external wallet applications
• Platform components (like Android's credential management) handle the logic
• Other trusted entities outside the UA manage credential flows

As noted in the PR #306 discussion, this is particularly relevant for Chrome where the logic lives in Android rather than the browser itself.

Solution

Updated the definition from:

A credential request client is a user-agent-defined component responsible for...

To:

A credential request client is a component responsible for managing the credential 
presentation or issuance flow, as defined or delegated by the user agent. The client 
may be implemented by the user agent itself, by an installed wallet, by platform-provided 
software, or by another trusted component.

Added clarifying note:

Note: The user agent may delegate some or all credential request client responsibilities 
to external wallet applications, platform components, or other trusted entities, according 
to user or platform policy.

Impact

This change ensures the specification supports both:

• UA-owned implementations (e.g., WebKit's direct wallet integration)
• Externally delegated implementations (e.g., Chrome delegating to Android platform, standalone wallet apps)

The user agent maintains its role in defining or delegating responsibilities while explicitly supporting diverse implementation patterns across different platforms and ecosystems.

Fixes the concern raised in PR #306 discussion threads.

Warning

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• esm.ubuntu.com
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[marcoscaceres]

Will integrate directly into other PR