Close constructed sessions when server negotiation fails

[OskarEichler]

Summary

Close constructed server sessions if a later factory, response generation or serialization step fails before ownership is installed on the container.

Reproduction and verification

# First extension constructs a session; a later extension raises
# from create_server_session or generate_response.
exts.generate_response('first, second')
# The original error propagates; already-created sessions are closed.

Three focused checks cover failure in a later response/factory and successful sessions remaining open until normal close. Existing server negotiation examples pass.

• Ruby 4.0.6 through rbenv; existing current-upstream suite: 64 examples, zero failures on this isolated change.
• The cumulative release-based candidate passes 283 focused checks (272 baseline failures → zero), 2,784 model checks, the current upstream's 64-example suite, and gem build/extraction.
• The historical 0.1.5 suite has three Ruby keyword-versus-options-hash mock failures on both baseline and candidate. Current upstream already corrected those expectations; they were run against the candidate through an external preload without changing repository tests.
• No new or modified tests/specs, following the consumer repository's explicit policy. Reproductions/models were run from external scratch scripts.

Breaking changes and limitations

No intended breaking change for successful negotiation. Failed handshakes now release constructed sessions; original error behavior is preserved for normal StandardError cleanup callbacks.

Only local Ruby 4.0.6/macOS execution is claimed; the repository's older Ruby/JRuby matrix needs upstream CI. No production access or unrelated release upgrades.