feat: complete multi-version protocol via field version registry

[mingri31164]

Complete the multi-version protocol task from previous PR by implementing the core field version registry mechanism with FieldVersionInitializer.

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Core Innovation - Field Version Registry

1. FieldVersionInitializer (NEW - Key Component)

• Pre-registers extension fields with their introduction versions
  • executeTimeOut, isAlarm, capacityAlarm → "2.0.0"
• Centralized version management for all fields
• Eliminates hardcoded version checks

2. FieldVersionRegistry (NEW)

• Global static registry: fieldName → introducedVersion
• Thread-safe ConcurrentHashMap implementation
• putIfAbsent ensures earliest version wins
• Persistent across requests

3. IncrementalContentUtil (Enhanced)

• Version-aware filtering based on registry
• Core parameters: always visible (bypass version check)
• Extension fields: filtered by clientVersion >= fieldVersion
• Dynamic field discovery with fallback to current server version

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How It Works

1. Server Startup:
   FieldVersionInitializer registers known fields to FieldVersionRegistry
   Example: registerField("executeTimeOut", "2.0.0")

2. Client Request (e.g., Client 1.5.0):
   → IncrementalContentUtil.getVersionedContent(param, "1.5.0")
   → resolveFieldRules() queries FieldVersionRegistry
   → executeTimeOut requires "2.0.0" but client is "1.5.0"
   → Field filtered out → No invalid refresh ✅

3. Config Update:
   → CacheItem.clearVersionMd5() clears cache
   → getMd5() returns null (FIXED: was returning full MD5)
   → Triggers recalculation with correct version filtering

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Critical Fixes

1. FieldVersionInitializer - The Missing Piece
   • Previous PR lacked centralized field version management
   • Fields were hardcoded or version-unaware
   • Now: All extension fields explicitly registered with versions

2. Cache Refresh Bug (Production Critical)
   • CacheItem.getMd5(): getOrDefault(key, fullMd5) → get(key)
   • Old clients no longer get full MD5 after cache clear
   • Prevents invalid refresh loop ✅

3. Code Quality
   • Extracted DEFAULT_SERVER_VERSION constant
   • Standardized comments in core files
   • Removed redundant auto-discovery mechanism

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Verification

• ✅ Requirement 1: Multi-field in single version (5 fields in 2.0.0)
• ✅ Requirement 2: Cross-version comparison (tested 1.5→2.2)
• ✅ 22 tests passing (IncrementalContentUtilTest, FieldVersionRegistryTest, etc.)

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Core Files

• FieldVersionInitializer.java (NEW) - Registers field versions at startup
• FieldVersionRegistry.java (NEW) - Global version registry
• IncrementalContentUtil.java - Version-aware content filtering
• CacheItem.java (FIXED) - Cache refresh logic
• ConfigCacheService.java - Uses incremental MD5