Add redundant-error-assertion checker for implied assert.Error / require.Error assertions

[mmorel-35]

This adds a new checker to catch redundant assert.Error* / require.Error* calls when a stronger error assertion on the same error expression already exists in the same block. The goal is to remove duplicate signal and keep error assertions minimal, regardless of whether the code uses assert or require.

• New checker: redundant-error-assertion

  • Introduces an advanced checker that scans assertion calls per ast.BlockStmt.
  • Reports redundant assert.Error* / require.Error* when paired with a stronger assertion on the same error expression:
    • ErrorContains*
    • ErrorIs*
    • ErrorAs*
    • EqualError*
  • Uses expression-string matching (analysisutil.NodeString) for same-error detection, with reassignment safety checks preserved.
  • Suggests an autofix that removes the redundant statement.

• Checker integration

  • Registers redundant-error-assertion in checker priority order and enables it by default.
  • Updates registry/factory expectations in checker/analyzer tests.

• Docs and examples

  • Adds redundant-error-assertion to the README checker table.
  • Adds a dedicated README section with invalid/valid patterns and autofix/default metadata.

• Analyzer test coverage

  • Adds checker-specific testdata (.go + .golden) for:
    • assert.Error* + stronger assertion on the same error
    • require.Error* + stronger assertion on the same error
    • mixed assert/require cases
    • assert object style (assObj.Error)
    • require object style (reqObj.Error)
    • non-matching cases such as different vars, different blocks, and standalone checks

// before
assert.Error(t, err)
require.ErrorContains(t, err, "not found")

// after
require.ErrorContains(t, err, "not found")