fix(channel): preserve UTF-8 across incremental event reads

[cyq1017]

Problem

The Core and CLI channel watchers decoded each appended buffer independently. A read boundary inside a multibyte UTF-8 character therefore produced replacement characters while leaving the JSON event parseable.

Change

Keep a streaming StringDecoder in each watcher's read state, reset it when the event file disappears or truncates, and cover both live implementations with split-byte regression tests.

Verification

• Core focused watcher regression: 1 passed
• CLI focused watcher regression: 1 passed
• Core test suite: 347 passed, 1 skipped
• CLI test suite: 1,709 passed
• pnpm lint
• pnpm typecheck
• focused four-file Prettier check
• git diff --check

Closes #568.

Summary by CodeRabbit

• Bug Fixes

  • Fixed channel event watching so multi-byte UTF-8 characters remain intact when data arrives in partial chunks.
  • Improved handling of event files that are truncated, rotated, replaced, or recreated.

• Tests

  • Added coverage confirming complete message text is preserved across incremental reads in both CLI and core channel watchers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: c4ed9fe9-06fd-4a55-b229-b2a658f8a8aa

📥 Commits

Reviewing files that changed from the base of the PR and between 64e6636 and df674eb.

📒 Files selected for processing (4)

• packages/cli/src/commands/channel/store/watch.ts
• packages/cli/test/commands/channel-watch.test.ts
• packages/core/src/channel/internal/store/watch.ts
• packages/core/test/channel/watch.test.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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📝 Walkthrough

Walkthrough

The Core and CLI channel watchers now preserve UTF-8 characters that span incremental file-read boundaries. New tests verify correct reconstruction of the 中 character in both watchers.

Changes

Channel watcher UTF-8 handling

Layer / File(s)	Summary
Core watcher decoding and validation packages/core/src/channel/internal/store/watch.ts, packages/core/test/channel/watch.test.ts	The core watcher maintains a StringDecoder across reads, resets it when file state resets, and preserves split UTF-8 characters. Tests validate the complete 中 character.
CLI watcher decoding and validation packages/cli/src/commands/channel/store/watch.ts, packages/cli/test/commands/channel-watch.test.ts	The CLI watcher maintains and resets a StringDecoder during event-file changes. Tests validate split UTF-8 input across incremental writes.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to df674

This change preserves UTF-8 characters across incremental event reads and includes regression coverage for both watchers. No actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the primary change: preserving UTF-8 characters across incremental event reads in channel watchers.
Linked Issues check	✅ Passed	The changes satisfy issue #568. Both Core and CLI watchers use persistent StringDecoder state, reset it when required, and include regression tests for characters split across reads.
Out of Scope Changes check	✅ Passed	All implementation and test changes directly support the UTF-8 watcher corruption fix in issue #568. No unrelated changes are identified.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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