fix(sessions): cap transcript payloads to stop renderer OOM

[charlesvien]

Problem

Opening a task with a huge session transcript OOMs the renderer (V8 "young object promotion failed") and the app crash-loops on that route. A user hit this 46 times on one task across three app versions. Transcript entries are loaded at full fidelity: whole-file tool outputs, diffs, terminal dumps and pasted media all stay uncapped in renderer heap, and resume chains fetch and stringify two full transcripts.

Changes

• New capStoredEntryPayloads bounds every stored transcript entry at acquisition: strings inside tool rawInput/rawOutput/_meta and diff old/new text truncate at 100k chars with a marker (structure preserved so tool views still render), text blocks truncate the same way, and media blocks over 10M chars become a text placeholder instead of a broken truncated base64 blob.
• Applied at all entry acquisition points so downstream comparisons (resume overlap detection, hydration hashes) see identical shapes: local/S3 log parse, the cloud hydration fetches, cloud watcher pages and the SSE stream.
• Diagnostics so the next report pinpoints itself: a renderer heap watchdog logs each upward 512MB heap crossing with the active route (lands in the chromium log tail attached to crash events), and session loads log entry counts and content sizes.

How did you test this?

• New vitest coverage: sessionEntryCaps.test.ts (19 cases) and useRendererHeapWatchdog.test.ts.
• Full @posthog/core (2508) and @posthog/ui (2021) suites, typecheck on code, @posthog/core and @posthog/ui, biome lint packages/core with zero noRestrictedImports.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 0438991.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "log transcript sizes when loading sessio..." | Re-trigger Greptile}