Add session id to Codex runtime analytics events

[michaelmusson-openai]

Summary

• add grouped session_id to thread initialized, turn, turn steer, compaction, guardian review, and subagent thread analytics payloads
• carry session_id through reducer thread metadata from PR 20437 ThreadStart/ThreadResume responses and subagent emission call sites
• update analytics serialization/reducer tests for the new field

Validation

• cargo fmt --package codex-analytics --package codex-core
• cargo test -p codex-analytics --no-default-features
• attempted: cargo test -p codex-analytics -p codex-core responses_headers --no-default-features, blocked by missing host libcap/pkg-config dependency in codex-linux-sandbox build script

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