Add EffectConflictDetector for detecting policy effect conflicts

[Copilot]

Implements a new detector that identifies conflicts where a user has one effect (allow/deny) for an action while their role has the opposite effect for the same action.

Implementation

• New ModelDetector interface: Extends detector framework to support detectors requiring both Model and RoleManager access
• EffectConflictDetector: Detects explicit allow/deny conflicts between user-level and role-level policies
• Updated RunDetections(): Handles both Detector and ModelDetector interfaces via type assertion

Design

The detector is opt-in rather than default. In Casbin, explicit user policies intentionally override role policies, so effect conflicts are often legitimate (e.g., denying a user an action their role allows). Users wanting strict validation can enable it:

e.SetDetectors([]detector.Detector{
    detector.NewDefaultDetector(),
    detector.NewEffectConflictDetector(),
})
err := e.RunDetections()  // Returns error on conflict

Example Conflict

p, alice, data2, write, deny
p, admin, data2, write, allow
g, alice, admin

The detector flags this as: user 'alice' has 'deny' effect for (data2, write), but role 'admin' has 'allow' effect for the same action

Testing

• 11 test cases covering conflicts, non-conflicts, edge cases (multiple roles, default effects, nil validation)
• Integration test verifying enforcer usage
• No regressions in existing test suite

Original prompt

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This section details on the original issue you should resolve

<issue_title>[feature] add second default detector: EffectConflictDetector</issue_title>
<issue_description>this detector needs to detect effect conflict. like alice is allowed to do something, but the role of alice is denied to do the same thing. so one allow and one deny. they are conflict.

note the deny here should be explicitly deny effect</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [feature] add second default detector: EffectConflictDetector #1666

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[hsluoyz]

fix:

[golangci](https://github.com/casbin/casbin/actions/runs/20825958603/job/59828063024#step:4:31)
issues found
[golangci: detector/effect_conflict_detector.go#L82](https://github.com/casbin/casbin/pull/1668/files#annotation_43866264688)
error is not nil (line 79) but it returns nil (nilerr)
[golangci: detector/effect_conflict_detector.go#L128](https://github.com/casbin/casbin/pull/1668/files#annotation_43866264693)
`if subject == user` has complex nested blocks (complexity: 7) (nestif)
[golangci: enforcer_test.go#L808](https://github.com/casbin/casbin/pull/1668/files#annotation_43866264700)
File is not `goimports`-ed (goimports)
[golangci: detector/effect_conflict_detector_test.go#L28](https://github.com/casbin/casbin/pull/1668/files#annotation_43866264705)
File is not `goimports`-ed (goimports)
[golangci: detector/effect_conflict_detector.go#L43](https://github.com/casbin/casbin/pull/1668/files#annotation_43866264709)
File is not `goimports`-ed (goimports)