Replace assert checks in logprob.scan with explicit error handling

[swarnaliiiiii]

Summary

This PR replaces internal assert checks in logprob.scan with explicit, user-facing errors and adds early validation to prevent reaching those assertions.

Motivation

Users working with time series models that rely on Scan (e.g. via CustomDist) have reported hitting internal assertions during log-probability computation, resulting in uninformative AssertionErrors. This makes it difficult to understand why log-probability derivation failed.

A concrete example is discussed in the PyMC Discourse thread:
https://discourse.pymc.io/t/smooth-local-linear-trend-with-customdist/16968

What this PR does

• Adds an early guard in logprob_scan to explicitly reject scans with multiple random outputs, which are currently unsupported.
• Replaces internal assert statements in convert_outer_out_to_in with explicit RuntimeErrors that explain why log-probability derivation failed.

What this PR does NOT do

• This PR does not attempt to support multi-output scans.
• It does not change Scan semantics or PyTensor internals.
• It does not alter any supported behavior; it only improves failure modes.

Related issue

addresses #7780

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.44%. Comparing base (11d0f1b) to head (c75500d).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc/logprob/scan.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8095      +/-   ##
==========================================
- Coverage   90.89%   87.44%   -3.46%     
==========================================
  Files         123      123              
  Lines       19501    19526      +25     
==========================================
- Hits        17726    17074     -652     
- Misses       1775     2452     +677     

Files with missing lines	Coverage Δ
pymc/logprob/scan.py	87.33% <50.00%> (-7.14%)	⬇️

... and 22 files with indirect coverage changes

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