test(readable): port setEncoding consume matrix

[ronag]

Summary

• port the relevant readable-body regressions from fix(readable): keep body bytes that arrive after setEncoding() nodejs/undici#5620
• cover chunks buffered before and after setEncoding and before and after body-mixin startup
• cover partial UTF-8 decoder state plus bytes, arrayBuffer, blob, text, JSON, and a real Client response drained before text()

The current lower-fork runtime already passes these cases, so this is intentionally a test-only PR.

Validation

• Node.js 26.7.0
• test/readable.js: 26/26 tests
• focused Client Response body silently truncated when setEncoding() is used and the body arrives in multiple chunks nodejs/undici#5611 integration: 1/1
• ESLint and git diff --check

Draft pending the full exact-head matrix and fresh Copilot review.

[Copilot]

Pull request overview

This PR adds regression coverage around Readable/body-mixin consumption behavior when setEncoding() is applied before and during chunk arrival, specifically targeting multi-byte UTF-8 boundary cases and ensuring later .text()/.json()/etc consumption includes all buffered chunks.

Changes:

• Added new Readable unit tests covering setEncoding() interactions across chunk boundaries (including partial UTF-8 decoder state) and verifying correctness across bytes(), arrayBuffer(), blob(), text(), and json().
• Added a Client integration regression test (nodejs#5611) to ensure setEncoding('utf8') followed by deferred .text() consumption preserves a multi-chunk response body.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/readable.js	Adds regression tests for Readable body-mixin consumption with setEncoding() across chunk boundaries and multiple consumption methods.
test/client-request.js	Adds an integration regression test ensuring Client response bodies remain intact when setEncoding() is set before delayed .text() consumption.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.