[Task] Add connection pooling and isolated execution scopes support for PHP

[jeremyprime]

Description

Add client-instance pooling and isolated execution scopes, matching the existing implementation in Java, Python, Node, and Go (valkey-io/valkey-glide#6338). The pool/scope logic lives in the Rust core (glide-core/src/pool.rs, scope.rs) and FFI layer (ffi/src/pool_ffi.rs). This task covers the language binding, tests, and documentation.

Client-Instance Pool: Bounded pool of reusable connections with LIFO reuse, background warmup, connectivity probe on create, state reset on release (DISCARD + SELECT), and configurable acquire timeout.

Isolated Execution Scopes: Dedicated per-connection state for WATCH/MULTI/EXEC with cluster slot pinning, cross-slot rejection, compression/timeout inheritance from parent client, and zero-cost release when state is clean.

Checklist

• Add ClientPool class with create/acquire/release/borrow/close/metrics API
• Add PoolConfig class (max_size, min_idle, idle_timeout, request_timeout, acquire_timeout)
• Add IsolatedScope class with command dispatch via FFI send_scope_command
• Wire FFI calls to libglide_ffi (pool_create, try_acquire, acquire_blocking, release, destroy, scope_try_acquire, scope_execute, scope_release)
• Add integration tests: pool lifecycle, borrow+commands, timeout, concurrent access, metrics
• Add integration tests: scope WATCH/MULTI/EXEC, cross-slot rejection (cluster), state reset
• Add documentation/examples

Related

• feat: cross-language client pool and isolated execution scopes valkey-glide#6338
• RFC - Dedicated Connection Pooling valkey-glide#5815 (RFC)
• Design doc: docs/pooling-and-scopes.md in valkey-glide