Add require-len checker for fail-fast indexed access guards

[mmorel-35]

Adds a dedicated require-len checker that enforces fail-fast length guards before indexed collection access. require-error is left untouched and remains focused solely on error assertions.

• New require-len checker

  • Reports assert.Len* calls that guard indexed access but should use require instead
  • Reports assert.* calls that use indexed access without a prior require.Len / require.NotEmpty guard
  • Provides autofix that inserts require.Len(t, arr, N) before the assertion, using require.NotEmpty(t, arr) when only index 0 is accessed
  • The hasLenGuard check only accepts fail-fast (require.*) guards and verifies the asserted length is >= maxIndex+1

• Missing require import handled in autofix

  • When the file does not yet import github.com/stretchr/testify/require, the fix also inserts the import
  • Uses require as the qualifier; falls back to require1, require2, etc. if require is already taken as a local identifier

• Registered as enabled-by-default with dedicated README docs, analyzer fixtures, and golden autofix output

[ccoVeille]

While I like the feature it brings, I'm a bit annoyed by the complexity of the code.

I know this comment is not actionable, I'm sorry.

But I wanted to share my point of view