fix(response_cache): don't require redis when disabled and require it when enabled

[bnjjj]

Previously, the router would attempt to connect to Redis for response caching regardless of whether response caching was enabled or disabled. This could cause unnecessary connection attempts and configuration errors even when the feature was explicitly disabled.

Now, the router properly validates Redis configuration based on the response cache state:

When response caching is disabled: Redis configuration is not required and no connection attempts are made.

When response caching is enabled: Redis configuration is validated and required. If a subgraph has caching enabled but no Redis configuration, the router will return a clear error:

Error: you must have a redis configured either for all subgraphs or for subgraph "products"

This validation ensures that:

• You can disable response caching without needing to provide Redis configuration
• When response caching is enabled, all enabled subgraphs have proper Redis connectivity (either via global all configuration or per-subgraph configuration)
• Configuration errors are caught at startup with clear error messages

Example configuration that now works correctly:

response_cache:
  enabled: false  # Redis not required when disabled
  # …
  subgraph:
    all:
      # …
      enabled: false

response_cache:
  enabled: true
  # …
  subgraph:
    all:
      enabled: true
      # …
      redis:  
        urls:
          - redis://127.0.0.1:6379  # Required when enabled

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Checklist

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• PR description explains the motivation for the change and relevant context for reviewing
• PR description links appropriate GitHub/Jira tickets (creating when necessary)
• Changeset is included for user-facing changes
• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Metrics and logs are added³ and documented
• Tests added and passing⁴
  • Unit tests
  • Integration tests
  • Manual tests, as necessary

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Notes

Footnotes

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[apollo-librarian]

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Build ID: bafad3508253ac36673cb999
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[Copilot]

Pull request overview

This PR fixes the response cache plugin to properly validate Redis configuration based on whether caching is enabled or disabled, preventing unnecessary Redis connection attempts and configuration errors when the feature is explicitly disabled.

Key changes:

• Added conditional Redis connection logic that only requires and connects to Redis when caching is enabled
• Introduced validation that returns clear errors when caching is enabled but Redis configuration is missing
• Added comprehensive test coverage for various enabled/disabled configuration combinations

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
apollo-router/src/plugins/response_cache/plugin.rs	Added conditional logic to check plugin and subgraph enabled state before connecting to Redis, added validation to ensure Redis config exists when needed, refactored subgraph_enabled into static method, and added comprehensive tests for various configuration scenarios
apollo-router/src/plugins/response_cache/tests.rs	Refactored tests to use new create_subgraph_conf helper function and updated for_test signature to accept SubgraphConfiguration instead of raw HashMap
.changesets/fix_response_cache_redis_requirement.md	Added changeset documentation explaining the behavior change with examples and validation rules

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