Improve ModelRegistry item assignment error

[russlan23]

Description

Closes #162.

Adds an explicit ModelRegistry.__setitem__ error that directs users to the controlled add API. The regression verifies the message and confirms that a failed assignment leaves the registry unchanged.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Validation

• python -m pytest -q ccflow/tests/test_base_registry.py - 43 passed
• python -m ruff check ccflow/base.py ccflow/tests/test_base_registry.py - passed
• python -m ruff format --check ccflow/base.py ccflow/tests/test_base_registry.py - passed
• git diff --check - passed

A clean full-suite attempt could not install the complete optional development extra because the package download timed out; the repository CI can provide the full declared matrix.

Checklist

• Linting passes
• Full make test passes locally
• New tests added for new functionality
• Documentation updated (not applicable)
• Changelog / version bump (not applicable)

[russlan23]

I refreshed this branch against current main after the lazy-registry work in #256 caused an adjacent conflict. The resolution keeps upstream's new ModelRegistry.__contains__ behavior and the focused __setitem__ guard from this PR.\n\nValidation on the merged head:\n\n- ccflow/tests/test_base_registry.py: 59 passed, 2 skipped\n- Ruff check and format check on the two affected Python files: passed\n- git diff --check: passed\n\nThe updated head is a1ba0acd; DCO passes and GitHub reports the PR mergeable again.

[timkpaine]

guard from this PR.\n\nValidation on the merged head:\n\n- `ccflow/tests/test_base_re

Might need to have codex swap newlines for actual newlines 😉

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (1a186fd) to head (a1ba0ac).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #254   +/-   ##
=======================================
  Coverage   93.47%   93.48%           
=======================================
  Files         176      176           
  Lines       20319    20327    +8     
  Branches     1350     1350           
=======================================
+ Hits        18994    19002    +8     
  Misses       1052     1052           
  Partials      273      273           

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