Add tests

[clementtalleu]

Add some tests on several methodes

• count
• scanKeys
• errors

(+ improve the exception system)

[guillaumedelre]

Proposition : #136

test(client): add comprehensive unit and integration tests for RedisClient and PredisClient

• RedisClient: 64 unit tests covering all methods, error paths, criteria building
  (TAG/NUMERIC/rangeFilters), pipeline, search/customSearch/searchLike branches,
  dash-escaping, numberOfResults limit, getIndexInfo parsing
• PredisClient: 65 unit tests mirroring RedisClient coverage, adapted for Predis
  internals (executeRaw, __call dispatch, cursor-based scanKeys, searchLike non-array guard)
• Integration: 16 tests against real Redis for scanKeys, discard, jsonGetProperty,
  hSet/hget, jsonSet/jsonGet, expire/expireTime, hGetAllMultiple, jsonGetMultiple
• Install predis/predis ^2.0||^3.0 as dev dependency (was declared but missing from lock)