fix(redis): avoid poisoning Lua SHA cache on transient load failures

[twisti-dev]

Motivation

• The previous implementation cached a Mono<String> from script.scriptLoad(...).cache(), and Mono.cache() retains terminal errors which can permanently replay transient SCRIPT LOAD failures.
• Permanently caching a failed load can cause persistent unavailability of Lua-backed operations for a script ID until process restart, creating an availability/DoS risk.
• The change aims to preserve retryability after transient failures by only caching successfully loaded script SHAs.

Description

• Change scriptShas from ConcurrentHashMap<String, Mono<String>> to ConcurrentHashMap<String, String> so the map stores concrete SHA strings rather than Mono instances.
• Update getSha to return Mono.just(cachedSha) when present, or call script.scriptLoad(...) and store the SHA into scriptShas via doOnNext only after a successful load.
• Preserve existing execute behavior, including RedisNoScriptException removal of the cached SHA and the retry policy, so functionality is unchanged except for avoiding cached load errors.

Testing

• Attempted to compile the core module with ./gradlew --no-daemon -q :surf-redis-core:compileKotlin, but the build requires Java 25 while the environment provides Java 21, so compilation could not be completed.
• No other automated tests were run in this environment due to the JVM version limitation.

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Codex Task