Distinguish Precmd hooks with completion metadata from prompt-only hooks

[vorporeal]

Description

Stack PR 3/8; depends on #12853.

Context

Once the new fields begin reaching clients, the parser must distinguish three meaningfully different inputs. A new sharer's Precmd contains a complete completion identity; an older sharer's Precmd contains prompt metadata only; and a payload containing just one completion field is malformed. Treating those cases as one partially optional value would spread ambiguity into the model and risk using incomplete or prompt-only evidence as proof that a command completed.

Approach

This PR makes the distinction explicit at the parser boundary. A canonical PrecmdValue contains non-optional CompletionMetadata plus reusable PromptMetadata; a prompt with neither completion field becomes a separate prompt-only event; and a prompt with exactly one completion field is rejected. Both JSON and key-value decoding follow the same classification rule.

The prompt-only portion is also separated so BlockList, Block, HeaderGrid, restoration, and cached prompt paths do not need to know about completion evidence. Canonical completion metadata reaches TerminalModel, but this PR intentionally sends Precmd hooks with completion metadata and prompt-only Precmd hooks through the existing prompt behavior. Recovery is deferred until the normal mutation pipeline and lifecycle gate exist.

Review guidance

• Is the both/neither/partial classification exhaustive and consistent across JSON and key-value hooks?
• Can malformed or prompt-only payloads accidentally acquire default completion evidence?
• Does the broad PromptMetadata refactor preserve current prompt and lifecycle behavior despite changing many call sites?

Testing

• Added parser coverage for payloads with completion metadata, prompt-only payloads, malformed partial payloads, invalid payloads, and unknown-field payloads.
• Updated affected block, terminal-model, restoration, and shared-session coverage for the typed prompt metadata refactor.
• Final-stack validation: cargo check -p warp --tests, ./script/format, and git diff --check.

[vorporeal]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Enable terminal lifecycle recovery in dogfood #12859
• Recover missing CommandFinished hooks from Precmd completion metadata #12858
• Safely reconcile repeated and prompt-only Precmd hooks #12857
• Gate terminal lifecycle transitions with a coordinator #12856
• Centralize terminal lifecycle mutations #12855
• Distinguish Precmd hooks with completion metadata from prompt-only hooks #12854 👈 (View in Graphite)
• Add command completion metadata to Precmd #12853
• Document terminal block lifecycle hardening design #12852
• master

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