Render the full orchestration subtree in shared-session viewers

[danielpeng2]

Description

Stacked on #14856 (owner-side subtree adoption; server half: warpdotdev/warp-server#14159).

A shared-session viewer of a tree-root orchestrator only rendered the root's direct children: the viewer SSE, its replay, and the cold-start REST seed were all ancestor-scoped, and every discovered run was hard-attributed to the stream anchor. Grandchildren spawned by remote children never appeared in the viewer's pill bar.

With MultiLevelOrchestration enabled:

• Scope resolution: the viewer streamer determines during the cold-start seed whether the anchor is a tree root (its task has no parent_run_id). Root anchors seed via the root_run_id list filter and stream via subtree_root_run_id; mid-tree anchors — and any anchor whose root-ness can't be determined — keep the existing direct-children ancestor scope, so both stream shapes remain in the viewer path.
• Event generalization: ChildSpawned/ChildStatusChanged now carry the wire event's actual parent_run_id alongside the stream-anchor id, instead of implying every run is a direct child of the anchor. The drain loop also skips the anchor's own events, which subtree streams inherently include.
• Nested placement: the viewer model attaches each discovered run under its actual parent's placeholder (resolved from the fetched task metadata, with the event-carried parent as a fallback hint), parking rows that arrive before their parent and draining them once the parent's placeholder registers — mirroring the owner-side placement semantics from Adopt root-scoped subtree event streams for orchestration tree roots #14856. Direct children of the anchor behave exactly as before, as does everything when the flag is off or the server is old (parent_run_id absent ⇒ legacy anchor attribution).

Viewer cursor semantics are intentionally unchanged (the viewer-mode seed still merges descendant cursors; it only surfaces lifecycle state and the seed carries current statuses).

Linked Issue

Part of the remote nested-orchestration visibility work; no standalone GitHub issue.

Testing

• New streamer tests: subtree-scope seed recording/descendant tracking, and drain behavior on subtree streams (anchor-event skipping, cursor advancement, parent_run_id propagation on emissions).
• New viewer-model tests: grandchild placement under its actual parent's placeholder, and park-until-parent-registers drain ordering.
• Updated existing streamer/viewer tests for the widened seed signature and event payloads; 117 tests in the two touched modules pass.
• ./script/format and all three presubmit clippy invocations pass.
• I have manually tested my changes locally with ./script/run

End-to-end validation (viewer of a root seeing a remote-remote grandchild live) is planned together with #14856 against a locally built warp-server from the PR S branch.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE

Co-Authored-By: Warp Agent agent@warp.dev

[danielpeng2]

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Learn more

• Render the full orchestration subtree in shared-session viewers #14857 👈 (View in Graphite)
• Adopt root-scoped subtree event streams for orchestration tree roots #14856
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