Fix Google GenAI Text Embedding missing taskType

[jewoodev]

Summary

Forward the configured taskType into EmbedContentConfig so GoogleGenAiTextEmbeddingModel#call stops silently falling back to the SDK default.

Closes #5966.

• Why: buildEmbeddingRequest merges taskType into the runtime options correctly, but call never passes it to EmbedContentConfig.Builder, so every request goes out with the SDK's default task type regardless of what the user set.

• Fix: Pass options.getTaskType().name() to EmbedContentConfig.Builder#taskType(String) when a task type is set, mirroring the existing outputDimensionality block right above it.

Tests

Added regression coverage in GoogleGenAiTextEmbeddingModelTests for:

• an explicit per-request taskType
• the RETRIEVAL_DOCUMENT default from initializeDefaults()

./mvnw -pl models/spring-ai-google-genai-embedding test \
    -Dtest='GoogleGenAiTextEmbeddingModelTests'