Add tests and doc for the feature and that

[AlessandroMinoccheri]

andThat() has been part of the codebase since the AndThatShouldParser interface was introduced, but it had no dedicated tests and no documentation. This PR closes that gap.
Changes
Tests

tests/Unit/Rules/AndThatShouldTest.php — 7 unit tests covering the AND logic directly against ClassDescription objects:

class matching all conditions + should() fails → violation produced
class matching all conditions + should() passes → no violation
class matching only the first condition → should() skipped
class matching only the second condition → should() skipped
class matching neither condition → should() skipped
triple-chained andThat() — all match → violation
triple-chained andThat() — last condition not met → should() skipped
reusing a base rule with different andThat() branches produces independent rules

tests/Integration/AndThatFilterTest.php — 4 integration tests using vfsStream + TestRunner with real PHP files parsed on disk:

namespace AND naming convention filter
namespace AND attribute filter
three chained andThat() conditions
except() respected alongside andThat()

Documentation

README.md — new Combining multiple conditions with andThat() section added after the except() section, covering: basic usage, the skipping behaviour on partial matches, combining with except(), and the independent-branch pattern when reusing a partially-built rule.

Notes
No production code changes. No breaking changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (8968da5) to head (0c8f684).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #616   +/-   ##
=========================================
  Coverage     98.33%   98.33%           
  Complexity      693      693           
=========================================
  Files            87       87           
  Lines          1981     1981           
=========================================
  Hits           1948     1948           
  Misses           33       33           

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