Restore CSP enum basket compatibility

[timkpaine]

csp-gateway 2.9 changed static dict-basket key detection to Python enum.Enum, which dropped keys for existing csp.Enum channel baskets. This restores support for both enum families.

Also updates GatewayTestHarness unset assertions to use Pydantic GatewayStruct field-presence semantics instead of hasattr(), including indexed list assertions.

Validation:

• make lint
• channel and harness tests pass
• reproduced with a downstream application using CSP enum-keyed baskets

[github-actions]

Test Results

853 tests  +2   843 ✅ +2   7m 3s ⏱️ -15s
  1 suites ±0    10 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 5762d20. ± Comparison against base commit 4f41f3e.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 72.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.59%. Comparing base (4f41f3e) to head (5762d20).

Files with missing lines	Patch %	Lines
csp_gateway/testing/harness.py	40.00%	6 Missing ⚠️
csp_gateway/server/gateway/csp/channels.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
- Coverage   86.60%   86.59%   -0.01%     
==========================================
  Files         156      156              
  Lines       18410    18428      +18     
  Branches     1742     1744       +2     
==========================================
+ Hits        15944    15958      +14     
- Misses       2465     2468       +3     
- Partials        1        2       +1     

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[timkpaine]

Superseded by #322; the compatibility changes were integrated into that branch.