stream: use validateString for consumer encoding

[sjungwon03]

This replaces the manual options.encoding type check in stream iterator
consumers with the shared validateString() validator.

- if (typeof options.encoding !== 'string') {
-   throw new ERR_INVALID_ARG_TYPE('options.encoding', 'string',
-                                  options.encoding);
- }
+ validateString(options.encoding, 'options.encoding');

This preserves the existing error behavior while using the common validation
helper. It also adds coverage for non-string encoding values in both the
async text() and sync textSync() consumers.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (4a5eb1c) to head (396f764).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64754   +/-   ##
=======================================
  Coverage   90.15%   90.15%           
=======================================
  Files         744      744           
  Lines      242517   242539   +22     
  Branches    45688    45690    +2     
=======================================
+ Hits       218642   218671   +29     
- Misses      15358    15368   +10     
+ Partials     8517     8500   -17     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/consumers.js	96.63% <100.00%> (-0.02%)	⬇️

... and 29 files with indirect coverage changes

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[mertcanaltin]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75251/