stream: fix merge handling for object-like sources

[trivikr]

merge() previously treated any final non-iterable object as an options
object, which dropped valid inputs that from() accepts, including
ArrayBuffer, ArrayBufferView, and objects implementing
Stream.toStreamable or Stream.toAsyncStreamable.

This updates the options-object heuristic to preserve those valid source
types and adds regression coverage for the affected cases.

Fixes: #63355

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.05%. Comparing base (2d6cbea) to head (2502bfe).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63356      +/-   ##
==========================================
- Coverage   90.06%   90.05%   -0.01%     
==========================================
  Files         714      714              
  Lines      225502   225524      +22     
  Branches    42628    42639      +11     
==========================================
+ Hits       203089   203090       +1     
- Misses      14195    14218      +23     
+ Partials     8218     8216       -2     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/consumers.js	96.61% <100.00%> (+0.05%)	⬆️

... and 44 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73495/