feat(unique): add #[Unique] attribute for single-field and composite unique constraints

[guillaumedelre]

Ref: #142

Enforce uniqueness at flush() time using Redis WATCH + MULTI/EXEC, for both single-field (property-level) and composite (class-level, IS_REPEATABLE) constraints.

Implementation:

• New #[Unique] attribute (TARGET_PROPERTY | TARGET_CLASS | IS_REPEATABLE)
• New UniqueConstraintViolationException with forField(), forFields(), concurrentModification() factory methods
• RedisObjectManager.flush() split into a WATCH path (when unique constraints are present) and the original path (no constraints)
• buildUniqueConstraintPlan() collects unique keys to WATCH, GET-check, SET, and DEL across all pending operations (persist, merge, delete)
• Intra-flush collision detection: duplicate values in the same flush() call are caught before touching Redis
• ObjectToPersist.previousValues stores the pre-merge snapshot so flush() can resolve which unique key to delete when a unique field changes
• AbstractPersister.delete() now passes value: $object (was null, caused ReflectionClass crash when building the unique key on delete)
• RedisClientInterface extended with get(), set(), watch(), unwatch(); exec() return type changed from void to bool (false = WATCH triggered)
• Both RedisClient (ext-redis) and PredisClient implementations updated

Unique key format:
unique:{className}:{field}:{value} (single-field)
unique:{className}:{f1,f2}:{v1}:{v2} (composite, fields sorted)

Tests:

• 17 functional tests (UniqueConstraintTest): persist/same-flush/merge/remove scenarios for both Hash and Json formats, single-field and composite
• 6 unit tests (UniqueConstraintViolationExceptionTest)

Docs:

• docs/mapping.md: new "Unique constraints" section (single-field, composite, merge/remove behavior, concurrency guarantee, limitations)
• docs/advanced_usage.md: exception handling snippet with link to mapping.md

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 79.74684% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.43%. Comparing base (5efb773) to head (d96617a).

Files with missing lines	Patch %	Lines
src/Om/RedisObjectManager.php	80.34%	23 Missing ⚠️
src/Client/RedisClient.php	18.18%	9 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff              @@
##               main     #143      +/-   ##
============================================
- Coverage     88.10%   87.43%   -0.67%     
- Complexity      900      949      +49     
============================================
  Files            68       70       +2     
  Lines          2076     2221     +145     
============================================
+ Hits           1829     1942     +113     
- Misses          247      279      +32     

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