Remove deprecated steps kwarg from GenerationStrategy (#5142)

[saitcakmak]

Summary:

GenerationStep(...) already returns a GenerationNode (it's a factory class using __new__). The steps= kwarg on GenerationStrategy.__init__ was a deprecated alias for nodes= that was scheduled for removal in early 2026.

This diff:

• Removes the steps= kwarg and **kwargs from GenerationStrategy.__init__, leaving only nodes: list[GenerationNode]
• Removes the associated deprecation warning and XOR validation logic
• Updates all 22 call sites across production code, tests, benchmarks, and the JSON decoder to use nodes= instead of steps=
• Removes unused warnings and Any imports

Reviewed By: lena-kashtelyan

Differential Revision: D99453408

[meta-codesync]

@saitcakmak has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99453408.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.39%. Comparing base (53be145) to head (197957c).

Files with missing lines	Patch %	Lines
ax/storage/json_store/decoder.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5142   +/-   ##
=======================================
  Coverage   96.38%   96.39%           
=======================================
  Files         613      613           
  Lines       68295    68288    -7     
=======================================
- Hits        65829    65823    -6     
+ Misses       2466     2465    -1     

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