[WIP] Synchronize README.md diagrams with current public API surface

[Copilot]

• Add synchronization badge (date 2025-10-15) to README.md
• Add Algorithm property to CredentialDescription in all diagrams
• Add AdditionalResponseParameters and BindingCertificate to AcquireTokenResult in all diagrams
• Add Name property to MicrosoftEntraApplicationOptions in all diagrams
• Fix ExtraHeadersParameters -> ExtraHeaderParameters throughout
• Update ExtraQueryParameters and ExtraHeaderParameters to show IDictionary<string, string> type in all diagrams
• Mark HttpMethod as read-only (<>) in DownstreamApiOptionsReadOnlyHttpMethod
• Add IAuthorizationHeaderProvider interface to diagram (if applicable)
• Update Serializer/Deserializer signatures to show nullability (? marks)
• Add default value annotations where applicable
• Verify all changes match the public API surface

Original prompt

Synchronizes README.md Mermaid diagrams with current public API surface. Changelog: - Added Algorithm to CredentialDescription. - Added AdditionalResponseParameters and BindingCertificate to AcquireTokenResult. - Added Name property to MicrosoftEntraApplicationOptions. - Corrected property name ExtraHeadersParameters -> ExtraHeaderParameters and fixed types to IDictionary<string,string>. - Ensured ExtraQueryParameters / ExtraHeaderParameters displayed as dictionaries throughout. - Marked HttpMethod in DownstreamApiOptionsReadOnlyHttpMethod as read-only. - Added generic interface IAuthorizationHeaderProvider. - Added condensed vs expanded IDownstreamApi diagrams. - Added extensibility diagram (credential loaders & custom signed assertion providers). - Annotated default values and audience union behavior. - Updated serializer/deserializer signatures to show nullability. - Added synchronization badge (date 2025-10-15). No behavioral code changes; documentation only.

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