Use eye.Spy for assertion failure checks and remove requireFailure helpers

[nfisher]

Motivation

• Standardize how test failures are inspected by replacing the ad-hoc requireFailure helper with the existing eye.Spy test spy for clearer intent and better integration with hammy assertions.
• Eliminate duplicated helper code across multiple test files to reduce maintenance and accidental divergence.
• Remove now-unused imports that were only needed for the helper and streamline test files.

Description

• Added github.com/gogunit/gunit/eye imports to httpassert_test.go, jsonassert_test.go, and yamlassert_test.go and removed the requireFailure helper functions from those files.
• Replaced calls like requireFailure(t, result, "...") with the spy pattern: aSpy := eye.Spy(); assert := hammy.New(aSpy); assert.Is(result); aSpy.HadErrorContaining(t, "...") to assert failure messages.
• Removed now-unused imports (for example strings where applicable) and cleaned up the test files accordingly.

Testing

• Ran go test ./... across the repository and the test suite completed successfully.
• All modified tests that inspect assertion failure messages now use eye.Spy and passed when run under the test command.

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Codex Task