Fix Optional[FromContext[T]] classification and define its semantics

[timkpaine]

_parse_annotation only peeled the top-level Annotated, so Optional[FromContext[int]] (a Union on the outside) was silently misclassified as a regular parameter and every call failed with a misleading "cannot satisfy unbound regular parameter" error.

Detect FromContext/Lazy markers nested inside a top-level Optional and define the two spellings precisely:

• FromContext[Optional[int]]: contextual, required-in-context, value may be None.
• Optional[FromContext[int]]: contextual, optional; absent -> bound to None (an implicit None default synthesized in _analyze_flow_function).

FromContext[Optional[int]] = None is therefore equivalent to Optional[FromContext[int]], and an explicit default still wins. Distinct required-ness yields distinct config/cache identities via the existing has_function_default/function_default identity terms.

Reject nested Lazy (Optional[Lazy[int]]) and non-Optional unions carrying FromContext (e.g. Union[FromContext[int], str]) with clear messages.

Adds focused regression tests covering all call shapes, the consistency equivalences, and the rejection cases.

[github-actions]

Test Results

    1 files  ± 0      1 suites  ±0   2m 50s ⏱️ -5s
1 129 tests  - 20  1 127 ✅  - 20  2 💤 ±0  0 ❌ ±0 
1 135 runs   - 20  1 133 ✅  - 20  2 💤 ±0  0 ❌ ±0 

Results for commit 2147313. ± Comparison against base commit d35c55d.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 97.64706% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (nk/auto_deps_auto_callable_model@d11a398). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
ccflow/tests/test_flow_model_optional_context.py	96.61%	2 Missing ⚠️

Additional details and impacted files

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##             nk/auto_deps_auto_callable_model     #235   +/-   ##
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  Files                                       ?      160           
  Lines                                       ?    17835           
  Branches                                    ?     1153           
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  Hits                                        ?    16589           
  Misses                                      ?     1020           
  Partials                                    ?      226           

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