stream: use validateString for consumer encoding

[sjungwon03]

Use validateString() for options.encoding in stream iterator consumers.

Add coverage for non-string values in the async and sync text consumers.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.13%. Comparing base (4a5eb1c) to head (4913e0f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64754      +/-   ##
==========================================
- Coverage   90.15%   90.13%   -0.02%     
==========================================
  Files         744      744              
  Lines      242517   242514       -3     
  Branches    45688    45687       -1     
==========================================
- Hits       218642   218595      -47     
- Misses      15358    15420      +62     
+ Partials     8517     8499      -18     

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

... and 32 files with indirect coverage changes

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[mertcanaltin]

lgtm