Add Flow.model(context_type=, strict=False) subset/omnibus context

[timkpaine]

By default a declared context_type may now be an "omnibus" superset: every FromContext[...] parameter must exist on the context with a compatible type, but the context may carry extra fields the model does not consume. This matches the otherwise-permissive bag-of-types design and lets multiple models share one broad context.

• _validate_declared_context_type(..., strict=False) keeps the missing-field and type-compatibility checks but only enforces the "every required context field is a FromContext param" bijection when strict=True.
• Runtime: _validate_declared_context_values validates the consumed fields individually when the declared context has unconsumed required fields (subset mode); otherwise it constructs the whole declared context so its cross-field validators still run. No config/serialization change needed.
• Thread strict through flow_model and document it on Flow.model.

Updates the existing extra-required-field test to reflect the new default (allowed by default, rejected under strict=True) and adds focused tests for subset execution, strict rejection/acceptance, and the shared missing/type checks that apply in both modes.

[github-actions]

Test Results

    1 files  ±0      1 suites  ±0   2m 51s ⏱️ -1s
1 170 tests +6  1 168 ✅ +6  2 💤 ±0  0 ❌ ±0 
1 176 runs  +6  1 174 ✅ +6  2 💤 ±0  0 ❌ ±0 

Results for commit 75b5781. ± Comparison against base commit 51396bc.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 95.58824% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.02%. Comparing base (51396bc) to head (75b5781).

Files with missing lines	Patch %	Lines
ccflow/tests/test_flow_model_strict_context.py	94.23%	3 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##           nk/auto_deps_auto_callable_model     #236   +/-   ##
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  Coverage                             93.01%   93.02%           
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  Files                                   160      161    +1     
  Lines                                 17835    17897   +62     
  Branches                               1153     1157    +4     
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+ Hits                                  16589    16648   +59     
- Misses                                 1020     1023    +3     
  Partials                                226      226           

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