test: Add comprehensive edge case and validation tests for core Spring AI builders

[alxkm]

Adds comprehensive test coverage across 3 critical Spring AI builder components:

PromptTemplateBuilderTests:

• Whitespace validation - Ensures whitespace-only templates are rejected
• Variable handling - Tests empty variables, null values, missing/partial variables
• Template rendering - Validates complete variable substitution and empty string handling
• Error scenarios - Confirms proper error messages for missing template variables

FilterExpressionBuilderTests:

• Operator coverage - Tests all comparison operators
• Complex expressions - Validates nested groups, NOT operations, and mixed data types
• Edge cases - Handles null values, empty lists, special characters in keys
• Real-world scenarios - Tests complex filter conditions for vector store queries

DocumentBuilderTests:

• Input validation - Tests whitespace IDs, empty text, null media handling
• Metadata management - Validates key-value pairs, map overrides, complex data types
• Builder behavior - Tests text/metadata overwriting, ID generation priority
• Builder reuse - Ensures proper state management across multiple builds

These tests make Spring AI more reliable by testing prompt templates, search filters, and document handling - preventing crashes and errors in AI applications.