fix: prevent app-wide crash on Pi messages with string content

[lincooln-ai]

Summary

Opening a Pi session that stores system / developer messages with a string content crashes the whole UI:

Something went wrong
e.filter is not a function

The error boundary replaces the entire app, so the session list becomes unreachable until a reload. This PR fixes it once at the parse boundary (root cause) and once in the entry renderer (last line of defence).

Root cause

Pi persists some roles with message.content as a plain string instead of a Content[]:

{"type":"message","id":"...","timestamp":"...","message":{"role":"system","content":"..."}}

1. normalizeSessionEntry() returns type === 'message' entries untouched, unlike the Claude/Codex converters, which normalise their content (src/utils/session.ts).
2. SessionEntryRenderer then calls contentToText(entry.message.content) — i.e. .filter(...) — for role === "developer" | "system", and stripPreviewAssistantContent() in preview mode. A string has no .filter, so the throw unmounts the tree into ErrorBoundary.

TypeError: e.filter is not a function
    at contentToText (SessionEntryRenderer.tsx)
    at renderSessionEntry (SessionEntryRenderer.tsx)

Reproduction

1. Open a Pi session whose JSONL contains a message with a string body — e.g. any session written by a current Pi version that persists the system prompt.
2. Click it in the session list (Tauri desktop app or pi-session-cli + browser, same frontend).
3. Result: Something went wrong / e.filter is not a function.

Affected sessions are not rare: 4 of 55 sessions in my library have this shape, and the crash is deterministic per session.

Fix

• src/utils/session.ts — new normalizePiMessageEntry() / normalizePiContent(): coerce non-array message.content into Content[] while parsing, so every consumer (renderer, previews, stats, plugins) sees a single shape. Already-array content is returned untouched, so the hot path is unaffected.
• src/components/session-viewer/SessionEntryRenderer.tsx — new asContentParts() guarding contentToText(), stripPreviewAssistantContent() and the content props passed to UserMessage / AssistantMessage. This is needed because live entries from get_pi_agent_entries bypass the JSONL parser, and a renderer should never be able to blank the whole app on unexpected session data.

Tests

• src/utils/session.test.ts — parser invariant: string bodies become text parts, empty strings become [], arrays stay untouched, and no emitted entry carries a non-array body.
• src/components/session-viewer/SessionEntryRenderer.test.tsx (new) — renders system, developer, empty-string, user and assistant-preview entries whose content is a raw string.

Verified the tests are meaningful — with the source fix reverted, all six fail with the production error:

# with fix
Test Files  2 passed (2)
     Tests  21 passed (21)

# fix reverted
Test Files  2 failed (2)
     Tests  6 failed | 15 passed (21)
  TypeError: content.filter is not a function

npx tsc --noEmit is clean. Full frontend suite: 768 passed; the 4 failures (extensions/pi-session-bridge/src/kanban-store.test.ts, extensions/pi-session-bridge/src/tools.test.ts, src/hooks/app/useUpdateChecker.test.tsx) also fail on a clean tree and are unrelated to this change.

Notes

• No Rust changes — the frontend fix covers both the Tauri build and the pi-session-cli / headless web UI.
• Normalising at the parse boundary rather than sprinkling Array.isArray checks also protects computeStats, session previews and branch/tree views, which all assume Content[].

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