Fix fixture declarations not propagating to auto-included shared example groups

[ngan]

Summary

Fixes a bug where fixture declarations are invisible to examples inside shared example groups that are auto-included via config.include_context with metadata filters.

Root cause: When config.include_context "shared examples", :some_metadata is configured, RSpec creates those child groups during configuration — before the spec file's fixture call sets the declaration on the parent group's metadata. Since RSpec snapshots parent metadata into child groups at creation time (metadata.update(parent_metadata)), the child's metadata never includes the :fixture_kit_declaration key. The prepend_before(:example, DECLARATION_METADATA_KEY) hook never fires for examples in those groups, leaving @_fixture_kit_repository nil.

Fix: Replace the direct metadata[DECLARATION_METADATA_KEY] = declaration with update_inherited_metadata(DECLARATION_METADATA_KEY => declaration). This is RSpec's own mechanism for pushing metadata updates — it sets the metadata on the current group AND recursively propagates to all existing children and their examples.

Real-world impact: This bug affects any RSpec spec where shared examples are globally auto-included via metadata filters. For example, at Gusto, all GraphQL mutation specs auto-include authorization shared examples via config.include_context 'applies mutation authorization correctly', :subgraph. Any mutation spec that uses fixture hits this bug.

Changes

• lib/fixture_kit/rspec.rb: Replace direct metadata assignment with update_inherited_metadata to propagate fixture declarations to already-created child groups
• spec/dummy/spec/integration/fixture_kit_integration.rb: Add integration test reproducing the exact pattern — shared examples auto-included via config.include_context with metadata, host group declares fixture
• spec/integration/dummy_app_spec.rb: Add expected assertion markers for the new test (RSpec-only, since shared examples are an RSpec concept)
• spec/unit/rspec_entrypoint_spec.rb: Add update_inherited_metadata to the test stub

Test plan

• New integration test reproduces the bug (fails without fix, passes with fix)
• All 157 existing tests pass
• Host group examples still work alongside shared example group examples
• Verified fix against real-world Gusto zenpayroll spec with auto-included mutation authorization shared examples (26 examples, 0 failures)

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