Retain the CreateSession server certificate for the Session's lifetime

[kevinherron]

Part 4 5.7.3.1 binds ActivateSession client signatures and encrypted token secrets to the serverCertificate returned by CreateSession. The server verified and decrypted against the endpoint's current certificate or the carrying channel's, so a Session retained across a server certificate rotation could not be reactivated with credentials on a replacement SecureChannel.

Sessions now capture the CreateSession certificate, chain, and key pair at creation and use them for client signature verification, user token decryption, X509 token signature checks, and enhanced user-token key selection. A Session created on an unsecured channel still uses the carrying channel's certificate.

This is the server half of #1949, split out because it is an independent server correctness fix that benefits any client reactivating a retained Session after rotation. #1949 will be rebased onto this branch; its rotation integration test exercises credential reactivation against a live server and depends on this change.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with no actionable correctness or security defects identified in the changed behavior.

Summary

• Adds immutable per-session access to the original server certificate context.
• Uses retained material for ActivateSession client signatures, token-secret decryption, X.509 token signatures, and enhanced user-token key signing.
• Preserves carrying-channel behavior for sessions created without SecureChannel certificate material.
• Updates identity-validator tests and package-level lifecycle documentation.

Diagram

sequenceDiagram
    participant C as Client
    participant S as SessionManager
    participant SS as Session
    participant V as IdentityValidator
    C->>S: CreateSession over certificate A
    S->>SS: Capture certificate A, chain, and key pair
    S-->>C: CreateSessionResponse with certificate A
    Note over S,SS: Server rotates to certificate B
    C->>S: ActivateSession on replacement SecureChannel B
    S->>SS: Read original certificate A
    S->>S: Verify client signature using A and channel B bindings
    S->>V: Validate/decrypt user token
    V->>SS: Use retained certificate A and key pair
    V-->>S: Valid identity
    S-->>C: ActivateSessionResponse

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