refactor(component,ai.gemini): standardize file api timeout and use native embedding type

[pinglin]

Because

• File API timeouts were inconsistent across different media types (60s for images/audio/documents, 120s for videos)
• The text embeddings output was being manually converted from the native ContentEmbedding type to []float64, losing metadata and requiring unnecessary type conversion
• Inconsistent timeouts could cause issues with large file uploads that need more processing time

This commit

• Introduces FileAPITimeout constant (300 seconds) to standardize File API operation timeouts across all media types (images, audio, videos, documents)
• Changes TaskTextEmbeddingsOutput.Embedding type from []float64 to *genai.ContentEmbedding to preserve the native Gemini API structure
• Removes unnecessary float32-to-float64 conversion loop in textEmbeddings() execution function
• Adds TASK_BATCH placeholder in tasks.yaml for future batch processing support
• Fixes trailing whitespace in definition.yaml

---

Note

Unifies File API timeouts to a 300s constant across media processing and returns native ContentEmbedding for text embeddings, removing float conversion.

• Gemini component:
  • File API:
    • Add FileAPITimeout (300s) and apply to processMediaFile for images, audio, videos, and documents in pkg/component/ai/gemini/v0/common.go.
  • Text Embeddings:
    • Change TaskTextEmbeddingsOutput.Embedding from []float64 to *genai.ContentEmbedding in pkg/component/ai/gemini/v0/io.go.
    • Update textEmbeddings() in pkg/component/ai/gemini/v0/task_text_embeddings.go to return the native ContentEmbedding and remove float32→float64 conversion.

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