feat: add HeaderSessionType constant and set it in Session middleware

[klaidliadon]

No description provided.

[Copilot]

Pull request overview

This PR adds a new Session-Type response header to expose the session type information to clients. The session type is already tracked internally (in context and logs) and used for access control, but was not previously exposed in HTTP response headers. This enhancement allows client applications to inspect the determined session type for debugging or auditing purposes.

• Introduced a new constant HeaderSessionType for standardized header name usage
• Set the Session-Type response header in both code paths of the Session middleware (early return for SpecialKey and normal flow)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
common.go	Adds HeaderSessionType constant defining the "Session-Type" header name, following the same pattern as HeaderAccessKey
middleware.go	Sets the Session-Type response header in the Session middleware for both the SpecialKey early return path and the standard session type determination path

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