Add configurable model endpoint identity header for APIM policies

[Bionic711]

Summary

Add an admin-configurable model endpoint identity header that sends a stable, non-reversible per-user key with outbound model endpoint calls.

User Value

APIM and backend policies need a consistent user-specific value for counters, quotas, routing, and throttling without exposing raw user identifiers such as UPN, object ID, or tenant ID.

Proposed Behavior

• Admins can enable or disable the header globally.
• Admins can customize the header name while blocking reserved auth/protocol headers.
• Admins can choose the identity input used for the HMAC: UPN, UPN + tenant ID, object ID, or object ID + tenant ID.
• Individual model endpoints can inherit, force-enable, force-disable, or override header name/value behavior.
• SimpleChat sends only an HMAC-SHA256 digest, never raw user identity values.
• Background/workflow calls use the owning or initiating user where available; otherwise the header is omitted.

Acceptance Criteria

• Global admin settings control header enablement, name, and identity value type.
• Per-endpoint overrides are saved and applied at runtime.
• Configured model endpoint calls and legacy GPT/APIM model calls include the header when enabled and identity values are available.
• Reserved headers cannot be used as the custom identity header name.
• Functional and UI coverage validate HMAC stability, privacy, disablement, override behavior, and endpoint override serialization.
• Feature documentation explains setup, APIM usage, and limitations.

Notes

Implementation is underway in the current working branch for version 0.250.186.

[paullizer]

Shipped in v0.250.203 via #1251.

Release note entries:

• v0.250.203: Model Endpoint Identity Header

Full notes: https://microsoft.github.io/simplechat/explanation/release_notes/

Closing as completed. This issue stayed open after the fix merged because pull requests target Development while the repository default branch is main, so GitHub never applied the Fixes # keyword automatically.