fix(lib): give each thread its own VCDiffDecoder

[xwang1498]

The shared static decoder keeps per-decode state, so two channels decoding on different threads at the same time were corrupting each other's deltas (40018, then a re-attach). Use a ThreadLocal instead.

Use older syntax for backwards compatibility.

I can add tests if you'd like, but it felt like a lot of ceremony to verify a pretty clear fix.

Summary by CodeRabbit

• Bug Fixes
  • Improved reliability when processing compressed messages concurrently by providing each thread with its own decoder instance.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: f96dd6f4-5d1f-408d-855f-b6af8a08ed87

📥 Commits

Reviewing files that changed from the base of the PR and between 243a08c and 34350cb.

📒 Files selected for processing (1)

• lib/src/main/java/io/ably/lib/types/BaseMessage.java

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Walkthrough

BaseMessage now stores VCDiffDecoder in a ThreadLocal. Each thread lazily receives its own decoder, and vcdiffApply uses that decoder for decoding.

Changes

Thread-local VCDiff decoding

Layer / File(s)	Summary
Decoder initialization and use lib/src/main/java/io/ably/lib/types/BaseMessage.java	vcdiffDecoder is now a thread-local value with lazy initialization. vcdiffApply retrieves the decoder with get() before decoding.

Priority: ➖ Normal

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

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 34350

The decoder isolation change preserves recovery behavior, and no actionable merge-blocking risk remains.

🚥 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 3 functions across 1 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 and concisely describes the main change: assigning each thread its own VCDiffDecoder.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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_{A rabbit checks the decoder thread by thread
Each gets its own path where VCDiff is led
Lazy ears wake when decoding begins
No shared state tangles the spins
BaseMessage hops onward, neatly fed}

_{Comment @coderabbitai help to get the list of available commands.}