Frame the input_signature probe's parameter as a multi-element set (wala/ML#638)

[khatchad]

Follow-up to the merged #467. A @tf.function(input_signature=[(None,) int32])-decorated f passes its parameter to g (the FUT). What g receives depends on the execution mode, which a static analysis cannot determine:

• traced (the default): the signature governs, g receives (None,) int32;
• under run_functions_eagerly: the signature is ignored, g receives the call-site argument's (3,) int32.

So the sound type of g's parameter is the set {(None,), (3,)} int32 -- a plain union, no provenance categories (which is also why the retracted wala#634's tagged-set diagnosis was wrong, not just its justification).

The fixture documents both modes with no run_functions_eagerly call (the eager element is just the argument at the call site; Ariadne doesn't and shouldn't analyze the global/dynamic eager mode). The test pins the current argument-derived (3,) and spells out the sound set in the TODO toward wala#638, which is reframed accordingly.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.34%. Comparing base (36d8c48) to head (2f42a05).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #469   +/-   ##
=========================================
  Coverage     72.34%   72.34%           
  Complexity     3083     3083           
=========================================
  Files           287      287           
  Lines         21615    21615           
  Branches       3625     3625           
=========================================
  Hits          15637    15637           
  Misses         4506     4506           
  Partials       1472     1472           

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