2b: Parse session + sessions envelope items in parser

[ajianaz]

Parent: #232 · Blocked by: #236, #240

Scope

Add two new branches in parse_envelope_text() for session and sessions item types.

Changes

// In parse_envelope_text(), after the existing "event" and "transaction" branches:
} else if item_type == "session" {
    if let Ok(session) = serde_json::from_str::<SessionUpdate>(body_line) {
        session_updates.push(session);
    }
} else if item_type == "sessions" {
    if let Ok(aggregates) = serde_json::from_str::<SessionAggregates>(body_line) {
        session_aggregates.push(aggregates);
    }
}

Tasks

• Add "session" branch → parse SessionUpdate
• Add "sessions" branch → parse SessionAggregates
• Test: envelope with session + event together (per Sentry spec, SDK sends them in same envelope)
• Test: envelope with only aggregated sessions
• Test: malformed session JSON → graceful skip
• Verify existing event/transaction parsing still works after changes

Effort: ~1 hour