Add error-compare checker: detect err.Error() in assertions

[mmorel-35]

Implements the error-compare checker (issue #47). Using err.Error() in testify assertions compares error string representations — fragile, ignores the error chain, and bypasses dedicated testify error APIs.

Detected patterns

Bad	Suggested fix	Autofix
assert.Contains(t, err.Error(), "substr")	assert.ErrorContains(t, err, "substr")	✅
assert.Contains(t, err.Error(), sentinel.Error())	assert.ErrorIs(t, err, sentinel)	✅
assert.Equal(t, err.Error(), "msg")	assert.EqualError(t, err, "msg")	✅
assert.Equal(t, "msg", err.Error())	assert.EqualError(t, err, "msg")	✅ (args swapped)
assert.Equal(t, err, sentinel) (both implement error)	assert.ErrorIs(t, err, sentinel)	❌
assert.NotEqual(t, err, sentinel) (both implement error)	assert.NotErrorIs(t, err, sentinel)	❌

All f-suffixed variants are handled automatically.

Implementation

• helpers_error.go: Added isErrErrorCall() (detects x.Error() where x implements error) and implementsErrorIface() (type-based, not restricted to error-typed variables).
• error_compare.go: New RegularChecker handling Contains, Equal, and NotEqual cases. Guards against overlap with error-nil via isNil check on Equal(t, err, nil). No autofix is provided for the two-error Equal/NotEqual cases because ErrorIs/NotErrorIs are not symmetric and argument order cannot be safely inferred from types alone.
• Registry: Inserted between contains and error-nil, enabled by default.
• Testgen: Separate errored and golden templates; Contains and Equal(err.Error(), ...) cases are autofixed, while Equal/NotEqual two-error cases remain unchanged in the golden output.
• README: Checker documented and added to the summary table.

Also reverts an unrelated change from a previous session that added a "redundant assertion" detection to require-error.

Closes #47

[ccoVeille]

Thank you for the fix of what I reported you

[thespags]

@Antonboom , some numbers
Repo:

Language                     files          blank        comment           code
-------------------------------------------------------------------------------
Go                            1333          48443          22319         219186

Usages

• require.EqualError 153
• require.ErrorIs 97
• require.ErrorContains 398

I agree that I'd prefer EqualError or ErrorIs over ErrorContains, but for a large org, I'm okay allowing engineers to use ErrorContains.

Regardless of preference the err.Error() check would be great. A good compromise could be a follow-up to enable/disable banning require.ErrorContains. Thoughts?