fix: clarify TemplateExpressionSpec reload errors

[MilesCranmerBot]

Fixes #846.

• Detect pickled/reloaded TemplateExpressionSpec models
• Raise a targeted unsupported-reload error for predict/export paths
• Add regression coverage for the error message

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.21%. Comparing base (beaa405) to head (762cb0c).

Files with missing lines	Patch %	Lines
pysr/sr.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1210      +/-   ##
==========================================
- Coverage   94.26%   94.21%   -0.05%     
==========================================
  Files          21       21              
  Lines        1690     1694       +4     
==========================================
+ Hits         1593     1596       +3     
- Misses         97       98       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.