fix(oak_session): disable TLS resumption with client auth

[secsson]

Summary

• disable TLS session tickets on client-authenticated C++ server connections;
• add an mTLS regression test that counts outgoing session tickets; and
• document why C++ client-authenticated servers do not support session resumption.

Closes #5167

Problem

OakSessionTlsContext retains one server SSL_CTX, while NewSession() resolves current trust anchors and attaches the custom verifier to each new SSL. A TLS 1.3 ticket created under an earlier client-authentication policy can therefore resume without a new client certificate check after the policy changes.

This affects servers that keep a context alive while rotating client trust anchors or changing a stateful verifier decision. The supplied reproduction records that an old-ticket connection succeeds after revocation while the same client without the ticket is rejected. No production C++ consumer has been demonstrated.

Fix

Set SSL_OP_NO_TICKET on each server-side SSL when SSL_VERIFY_PEER is enabled. Applying the option per connection avoids mutating the shared context and leaves servers without client authentication unchanged.

The README now makes the behavior explicit: C++ servers requiring client authentication perform a fresh authentication exchange for each new connection so current trust anchors and verifier policy are applied.

Tests

• Added ClientAuthenticatedServerDoesNotIssueSessionTickets, which completes an mTLS handshake with a session-cache-capable client and counts outgoing server tickets.
• Negative control: with the fix removed, the new test fails because BoringSSL emits two NewSessionTicket messages.
• Fixed control: with the fix restored, the new test observes zero tickets.
• bazel test //oak_session/tls:oak_session_tls_test --test_output=errors passes.
• git diff --check passes.
• The original standalone policy-rotation PoC was not rerun because its files and logs were unavailable.

Compatibility

There is no public API or framing change. Servers without client authentication retain their existing behavior. Client-authenticated C++ servers intentionally lose TLS session-resumption performance and require a fresh certificate exchange on reconnect so revocation and policy rotation take effect.

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[jblebrun]

Thanks for this fix!

Please run just format on the change set (it should result in a small change in the README.md file).

[secsson]

Thanks for this fix!

Please run just format on the change set (it should result in a small change in the README.md file).

Ran just format and pushed the resulting README change. Thanks!