fix(interactive): treat missing thinking/text payloads as empty instead of crashing

[groovyplanet]

Summary

Provider extensions can emit assistant content blocks in the wrong shape
(e.g. { type: "thinking", text: ... } instead of { type: "thinking", thinking: ... },
or a bare block with no payload). The interactive TUI crashed with an uncaught

                                                                                                                                                                                                                                                                                                                                                                       
TypeError: Cannot read properties of undefined (reading 'length')                                                                                                                                                                                                                                                                                                      
                                                                                                                                                                                                                                                                                                                                                                       

during smooth streaming reveal (block.thinking.length), and transcript
rendering could also crash on already-stored messages (content.thinking.trim()).

This hardens the core renderer so malformed blocks degrade to empty content
instead of terminating the whole TUI process.

Changes

• streaming-reveal-content.ts: countVisibleUnits / buildDisplayMessage
  treat a missing text/thinking payload as "" before counting, slicing,
  and .length checks.
• assistant-render-descriptors.ts: isVisibleContent and descriptor
  extraction use (content.text ?? "").trim() / (content.thinking ?? "").trim().
• test/streaming-reveal-content.test.ts: regression test for malformed
  thinking/text blocks; fails with the exact production TypeError when the
  fix is reverted.
• changes.md: records the fork-visible behavior change.

Test plan

• RED: regression test fails with the exact production TypeError when the
  fix is reverted.
• GREEN: 19 reveal tests + 28 render-path tests pass.
• Root tsc --noEmit, biome check --error-on-warnings, and the repo's
  pre-commit gate (biome + pinned-deps + ts-imports + shrinkwrap + browser
  smoke) all pass.

---

Summary by cubic

Fixes a crash in the interactive TUI when provider extensions emit malformed assistant blocks. Missing text/thinking payloads are treated as empty strings so streaming reveal and transcript rendering don’t throw.

• Bug Fixes
  • Normalize missing text/thinking to "" in streaming reveal and render descriptor paths to avoid .length/.trim() on undefined.
  • Add a regression test for malformed blocks that previously caused “Cannot read properties of undefined (reading 'length')”.

^{Written for commit 2432d02. Summary will update on new commits.}

[groovyplanet]

Remaining unguarded read in the same crash path — follow-up suggestion

This PR hardens the two interactive render paths (streaming-reveal-content.ts, assistant-render-descriptors.ts), but the same crash signature still exists for persisted malformed blocks:

• packages/coding-agent/src/core/compaction/compaction.ts:339-341 — estimateTokens() reads block.text.length / block.thinking.length unguarded. A malformed block that survives into a session (e.g. { type: "thinking", text: ... } with no thinking key, or a bare { type: "text" }) throws the identical TypeError: Cannot read properties of undefined (reading 'length') when compaction estimates tokens over that session.

Two options for a follow-up:

1. Apply the same ?? "" guard in estimateTokens (minimal, same shape as this PR), or
2. Normalize/validate assistant content blocks at the ingestion boundary so every consumer (streaming, render, compaction, export) is safe at once.

Note: core/extensions/builtin/claude-sdk-oauth/prompt-bridge.ts:35 also calls block.text.trim() unguarded — worth auditing in the same pass.

Happy to open a separate PR for either approach.