Fix multi-output IfElse logp for non-lazy backends

[eclipse1605]

Description

in #8036, a multi output IfElse was being rewritten into separate single output ifelses, which:

• breaks cross output dependencies within a branch (e.g. output 2 depending on output 1), and
• can trigger runtime shape assertions (SpecifyShape) from the dead branch when execution is non lazy.

i tried to fix it by:

• handle it jointly as a measurable IfElse. computing per output logps per branch while substituting cross output dependencies with the corresponding value variables (so each output’s logp can be derived in a measurable IR).
• make logprob evaluation robust to non lazy backends by using "branch safe" value variables: the selected branch stays strict, while the non selected branch uses a shape coerced value to prevent dead branch shape assertions from crashing.
• remove SpecifyShape nodes during logprob IR cleanup, since they only add runtime assertions and can cause dead branch failures under non lazy evaluation.
• keep transform measurability detection from incorrectly treating deterministic wrappers as extra "sources of measurability" unless they contain RandomVariable ancestry, which is needed to keep the new IfElse logprob path measurable.

Related Issue

• Related to Make ifelse logp robust to non-lazy backends #8036

[codecov]

Codecov Report

❌ Patch coverage is 90.14085% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.69%. Comparing base (c68c56e) to head (a7a853f).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc/logprob/mixture.py	87.50%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8070      +/-   ##
==========================================
- Coverage   91.42%   90.69%   -0.74%     
==========================================
  Files         117      121       +4     
  Lines       19154    19521     +367     
==========================================
+ Hits        17512    17704     +192     
- Misses       1642     1817     +175     

Files with missing lines	Coverage Δ
pymc/logprob/rewriting.py	100.00% <100.00%> (ø)
pymc/logprob/transforms.py	95.91% <100.00%> (+0.27%)	⬆️
pymc/logprob/mixture.py	85.90% <87.50%> (-9.83%)	⬇️

... and 14 files with indirect coverage changes

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