Fix shared session scrollback restore

[zachbai]

Description

• Formalizes shared-session scrollback snapshots as completed historical blocks plus an optional unfinished active block.
• Restores completed-only snapshots without asserting that the final serialized block is active, and ensures the viewer model still has a fresh hidden active block.
• Preserves follow-up duplicate skipping and adds focused regression coverage.
• Adds specs/APP-4457/TECH.md documenting the crash, Sentry issue, contract, and validation strategy.

Linked Issue

• Linear: https://linear.app/warpdotdev/issue/APP-4457/fix-shared-session-scrollback-restore-crash-when-active-block-is
• Sentry: https://warpdotdev.sentry.io/issues/7475296168/?alert_rule_id=15217488&alert_type=issue&notification_uuid=93ccd349-0267-4363-90d3-925a1c8ca43f&project=5701177&referrer=slack
• The linked issue is labeled ready-to-spec or ready-to-implement. N/A: this is a newly-created Linear issue for the crash fix.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes). N/A: no UI change.

Testing

• cargo fmt

• cargo nextest run -p warp --lib test_loading_scrollback

• cargo nextest run -p warp --lib test_append_followup_scrollback

• cargo clippy --workspace --all-targets --all-features --tests -- -D warnings

• git --no-pager diff --check -- app/src/terminal/model/blocks.rs app/src/terminal/model/block.rs app/src/terminal/shared_session/mod.rs app/src/terminal/shared_session/mod_tests.rs app/src/terminal/shared_session/viewer/event_loop_tests.rs specs/APP-4457/TECH.md

• I have manually tested my changes locally with ./script/run

Screenshots / Videos

N/A — no user-visible UI change.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode
  Co-Authored-By: Oz oz-agent@warp.dev

[oz-for-oss]

@zachbai

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[zachbai]

• Fix shared session scrollback restore #10780 👈 (View in Graphite)
• master

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[oz-for-oss]

Overview

This PR updates shared-session scrollback restore to treat the active block as optional, restores completed-only scrollback without tripping the old active-block assertion, and adds regression coverage for initial and follow-up restore paths.

Concerns

• None identified in the changed lines.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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