stream: fix decoded fromList chunk boundary check

[watson]

Correct fromList() in decoded string mode to compare n against the current chunk length, not the buffer array length.

This prevents over-consuming chunks, which can corrupt readable state and crash with TypeError when mixing setEncoding() and read(n).

I reproduced this in all non-EoL release lines.

For reference, here's the script I used to make read() throw (usually it throws almost instantly, but a few times it can take a little while):

'use strict'

const https = require('node:https')

const url = process.argv[2] || 'https://www.google.com'
const maxRuns = Number(process.argv[3] || 2000)
let runs = 0

function runOnce () {
  runs++
  https.get(url, (res) => {
    res.setEncoding('utf8') // If this line is removed, the crash doesn't happen.

    res.on('readable', () => {
      // Race condition: The call to `res.read(100)` might throw
      while (res.read(100) !== null) {}
    })

    res.on('end', () => {
      if (runs >= maxRuns) {
        console.log(`completed ${runs} runs without crash`)
      } else {
        runOnce()
      }
    })
  })
}

console.log(`reproducing with ${url} for up to ${maxRuns} runs...`)
runOnce()

And here's the output:

reproducing with https://www.google.com for up to 2000 runs...
node:internal/streams/readable:1650
  } else if (n < buf[idx].length) {
                          ^

TypeError: Cannot read properties of undefined (reading 'length')
    at fromList (node:internal/streams/readable:1650:27)
    at Readable.read (node:internal/streams/readable:756:11)
    at IncomingMessage.<anonymous> (/Users/thomas.watson/go/src/github.com/DataDog/dd-trace-js/foo.js:16:18)
    at IncomingMessage.emit (node:events:508:20)
    at emitReadable_ (node:internal/streams/readable:837:12)
    at process.processTicksAndRejections (node:internal/process/task_queues:89:21)

Node.js v25.6.1

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71377/

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.73%. Comparing base (d5279e5) to head (2984f13).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/readable.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61884      +/-   ##
==========================================
- Coverage   91.81%   89.73%   -2.08%     
==========================================
  Files         338      675     +337     
  Lines      140073   204855   +64782     
  Branches    22081    39372   +17291     
==========================================
+ Hits       128605   183829   +55224     
- Misses      11242    13287    +2045     
- Partials      226     7739    +7513     

Files with missing lines	Coverage Δ
lib/internal/streams/readable.js	97.16% <0.00%> (+0.88%)	⬆️

... and 456 files with indirect coverage changes

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[mcollina]

lgtm