chore(feature_flags): warn on empty schema and empty rules

[dreamorosi]

Issue number: closes #8427

Summary

Changes

SchemaValidator now emits a warning log for documents that are valid but almost certainly not what the author meant. Nothing is raised, so every document that validated before still validates.

Three cases are covered:

• Empty top-level document ({}). Typically the result of an envelope query that matched nothing, e.g. a typo'd feature group name.
• A feature whose rules key is present but empty ({} or None).
• A feature whose rules key is present, empty, and not a dictionary ([], ""). This gets a more specific message because the same value, once populated, would be rejected with SchemaValidationError; the empty form only slipped through because if not self.rules short-circuits first.

Omitting rules entirely stays silent, since that is the documented way to declare a static flag.

While wiring the warning I found that RulesValidator derived feature_name from next(iter(self.feature)), i.e. the feature's first key ("default" in practice), not its name. FeaturesValidator now passes the real name in through a new optional feature_name argument, so the new warnings and the existing Feature rules must be a dictionary, feature=... error name the right feature. The old fallback is kept so direct construction without the argument keeps working.

Tests pin each warning's level and message, and pin the two silent paths (well-formed document, rules omitted) so the warnings do not become noisy.

The TypeScript port (aws-powertools/powertools-lambda-typescript#5614) warns on the same cases and throws only where Python throws, so documents stay portable between runtimes.

User experience

No behaviour change for evaluate or get_enabled_features. The only difference is in logs:

Document	Before	After
{}	silent	WARNING Feature flags schema is empty, no features to validate
{"f": {"default": false, "rules": {}}}	silent	WARNING Feature has 'rules' but it is empty, feature=f
{"f": {"default": false, "rules": []}}	silent	WARNING Feature 'rules' should be a dictionary but is an empty list, feature=f
{"f": {"default": false}}	silent	silent
{"f": {"default": false, "rules": "4"}}	raises	raises, now with feature=f instead of feature=default

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.65%. Comparing base (a39e101) to head (c4cd833).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8430   +/-   ##
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  Coverage    96.65%   96.65%           
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  Files          296      296           
  Lines        14767    14774    +7     
  Branches      1246     1249    +3     
========================================
+ Hits         14273    14280    +7     
  Misses         359      359           
  Partials       135      135           

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